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

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

Fixes: 9e264f3f85a5 ("spi: Replace all spi->chip_select and spi->cs_gpiod references with function call")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20241023203032.1388491-1-Frank.Li@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>