scsi: myrs: Fix dma_alloc_coherent() error check

Check for NULL return value with dma_alloc_coherent(), because DMA
address is not always set by dma_alloc_coherent() on failure.

Fixes: 77266186 ("scsi: myrs: Add Mylex RAID controller (SCSI interface)")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Link: https://lore.kernel.org/r/20250725083112.43975-2-fourier.thomas@gmail.com


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>