spi: spi-fsl-dspi: Fix crash when not using GPIO chip select

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

spi: spi-fsl-dspi: Fix crash when not using GPIO chip select

Add check for the return value of spi_get_csgpiod() to avoid passing a NULL
pointer to gpiod_direction_output(), preventing a crash when GPIO chip
select is not used.

Fix below crash:
[ 4.251960] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
[ 4.260762] Mem abort info:
[ 4.263556] ESR = 0x0000000096000004
[ 4.267308] EC = 0x25: DABT (current EL), IL = 32 bits
[ 4.272624] SET = 0, FnV = 0
[ 4.275681] EA = 0, S1PTW = 0
[ 4.278822] FSC = 0x04: level 0 translation fault
[ 4.283704] Data abort info:
[ 4.286583] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 4.292074] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 4.297130] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 4.302445] [0000000000000000] user address but active_mm is swapper
[ 4.308805] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[ 4.315072] Modules linked in:
[ 4.318124] CPU: 2 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.12.0-rc4-next-20241023-00008-ga20ec42c5fc1 #359
[ 4.328130] Hardware name: LS1046A QDS Board (DT)
[ 4.332832] pstate: 40000005 (nZcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 4.339794] pc : gpiod_direction_output+0x34/0x5c
[ 4.344505] lr : gpiod_direction_output+0x18/0x5c
[ 4.349208] sp : ffff80008003b8f0
[ 4.352517] x29: ffff80008003b8f0 x28: 0000000000000000 x27: ffffc96bcc7e9068
[ 4.359659] x26: ffffc96bcc6e00b0 x25: ffffc96bcc598398 x24: ffff447400132810
[ 4.366800] x23: 0000000000000000 x22: 0000000011e1a300 x21: 0000000000020002
[ 4.373940] x20: 0000000000000000 x19: 0000000000000000 x18: ffffffffffffffff
[ 4.381081] x17: ffff44740016e600 x16: 0000000500000003 x15: 0000000000000007
[ 4.388221] x14: 0000000000989680 x13: 0000000000020000 x12: 000000000000001e
[ 4.395362] x11: 0044b82fa09b5a53 x10: 0000000000000019 x9 : 0000000000000008
[ 4.402502] x8 : 0000000000000002 x7 : 0000000000000007 x6 : 0000000000000000
[ 4.409641] x5 : 0000000000000200 x4 : 0000000002000000 x3 : 0000000000000000
[ 4.416781] x2 : 0000000000022202 x1 : 0000000000000000 x0 : 0000000000000000
[ 4.423921] Call trace:
[ 4.426362] gpiod_direction_output+0x34/0x5c (P)
[ 4.431067] gpiod_direction_output+0x18/0x5c (L)
[ 4.435771] dspi_setup+0x220/0x334