firmware: arm_scmi: Fix double free in SMC transport cleanup path

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

firmware: arm_scmi: Fix double free in SMC transport cleanup path

When the generic SCMI code tears down a channel, it calls the chan_free
callback function, defined by each transport. Since multiple protocols
might share the same transport_info member, chan_free() might want to
clean up the same member multiple times within the given SCMI transport
implementation. In this case, it is SMC transport. This will lead to a NULL
pointer dereference at the second time:

| scmi_protocol scmi_dev.1: Enabled polling mode TX channel - prot_id:16
| arm-scmi firmware:scmi: SCMI Notifications - Core Enabled.
| arm-scmi firmware:scmi: unable to communicate with SCMI
| Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
| Mem abort info:
| ESR = 0x0000000096000004
| EC = 0x25: DABT (current EL), IL = 32 bits
| SET = 0, FnV = 0
| EA = 0, S1PTW = 0
| FSC = 0x04: level 0 translation fault
| Data abort info:
| ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
| CM = 0, WnR = 0, TnD = 0, TagAccess = 0
| GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
| user pgtable: 4k pages, 48-bit VAs, pgdp=0000000881ef8000
| [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
| Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
| Modules linked in:
| CPU: 4 PID: 1 Comm: swapper/0 Not tainted 6.7.0-rc2-00124-g455ef3d016c9-dirty #793
| Hardware name: FVP Base RevC (DT)
| pstate: 61400009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
| pc : smc_chan_free+0x3c/0x6c
| lr : smc_chan_free+0x3c/0x6c
| Call trace:
| smc_chan_free+0x3c/0x6c
| idr_for_each+0x68/0xf8
| scmi_cleanup_channels.isra.0+0x2c/0x58
| scmi_probe+0x434/0x734
| platform_probe+0x68/0xd8
| really_probe+0x110/0x27c
| __driver_probe_device+0x78/0x12c
| driver_probe_device+0x3c/0x118
| __driver_attach+0x74/0x128
| bus_for_each_dev+0x78/0xe0
| driver_attach+0x24/0x30
| bus_add_driver+0xe4/0x1e8
| driver_register+0x60/0x128
| __platform_driver_register+0x28/0x34
| scmi_driver_init+0x84/0xc0
| do_one_initcall+0x78/0x33c
| kernel_init_freeable+0x2b8/0x51c
| kernel_init+0x24/0x130
| ret_from_fork+0x10/0x20
| Code: f0004701 910a0021 aa1403e5 97b91c70 (b9400280)
| ---[ end trace 0000000000000000 ]---

Simply check for the struct pointer being NULL before trying to access
its members, to avoid this situation.

This was found when a transport doesn't really work (for instance no SMC
service), the probe routines then tries to clean up, and triggers a crash.