pinctrl: mcp23s08: Disable all pin interrupts during probe

In the Linux kernel, the following vulnerability has been resolved:

pinctrl: mcp23s08: Disable all pin interrupts during probe

A chip being probed may have the interrupt-on-change feature enabled on
some of its pins, for example after a reboot. This can cause the chip to
generate interrupts for pins that don't have a registered nested handler,
which leads to a kernel crash such as below:

[ 7.928897] Unable to handle kernel read from unreadable memory at virtual address 00000000000000ac
[ 7.932314] Mem abort info:
[ 7.935081] ESR = 0x0000000096000004
[ 7.938808] EC = 0x25: DABT (current EL), IL = 32 bits
[ 7.944094] SET = 0, FnV = 0
[ 7.947127] EA = 0, S1PTW = 0
[ 7.950247] FSC = 0x04: level 0 translation fault
[ 7.955101] Data abort info:
[ 7.957961] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 7.963421] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 7.968447] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 7.973734] user pgtable: 4k pages, 48-bit VAs, pgdp=00000000089b7000
[ 7.980148] [00000000000000ac] pgd=0000000000000000, p4d=0000000000000000
[ 7.986913] Internal error: Oops: 0000000096000004 [#1] SMP
[ 7.992545] Modules linked in:
[ 8.073678] CPU: 0 UID: 0 PID: 81 Comm: irq/18-4-0025 Not tainted 7.0.0-rc6-gd2b5a1f931c8-dirty #199
[ 8.073689] Hardware name: Khadas VIM3 (DT)
[ 8.073692] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 8.094639] pc : _raw_spin_lock_irq+0x40/0x80
[ 8.098970] lr : handle_nested_irq+0x2c/0x168
[ 8.098979] sp : ffff800082b2bd20
[ 8.106599] x29: ffff800082b2bd20 x28: ffff800080107920 x27: ffff800080104d88
[ 8.106611] x26: ffff000003298080 x25: 0000000000000001 x24: 000000000000ff00
[ 8.113707] x23: 0000000000000001 x22: 0000000000000000 x21: 000000000000000e
[ 8.120850] x20: 0000000000000000 x19: 00000000000000ac x18: 0000000000000000
[ 8.135046] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
[ 8.135062] x14: ffff800081567ea8 x13: ffffffffffffffff x12: 0000000000000000
[ 8.135070] x11: 00000000000000c0 x10: 0000000000000b60 x9 : ffff800080109e0c
[ 8.135078] x8 : 1fffe0000069dbc1 x7 : 0000000000000001 x6 : ffff0000034ede00
[ 8.135086] x5 : 0000000000000000 x4 : ffff0000034ede08 x3 : 0000000000000001
[ 8.163460] x2 : 0000000000000000 x1 : 0000000000000001 x0 : 00000000000000ac
[ 8.170560] Call trace:
[ 8.180094] _raw_spin_lock_irq+0x40/0x80 (P)
[ 8.184443] mcp23s08_irq+0x248/0x358
[ 8.184462] irq_thread_fn+0x34/0xb8
[ 8.184470] irq_thread+0x1a4/0x310
[ 8.195093] kthread+0x13c/0x150
[ 8.198309] ret_from_fork+0x10/0x20
[ 8.201850] Code: d65f03c0 d2800002 52800023 f9800011 (885ffc01)
[ 8.207931] ---[ end trace 0000000000000000 ]---

This issue has always been present, but has been latent until commit
"f9f4fda15e72" ("pinctrl: mcp23s08: init reg_defaults from HW at probe and
switch cache type"), which correctly removed reg_defaults from the regmap
and as a side effect changed the behavior of the interrupt handler so that
the real value of the MCP_GPINTEN register is now being read from the chip
instead of using a bogus 0 default value; a non-zero value for this
register can trigger the invocation of a nested handler which may not exist
(yet).
Fix this issue by disabling all pin interrupts during initialization.