scsi: qedf: Fix potential dereference of NULL pointer

The return value of dma_alloc_coherent() needs to be checked to avoid use
of NULL pointer in case of an allocation failure.

Link: https://lore.kernel.org/r/20211216101449.375953-1-jiasheng@iscas.ac.cn


Fixes: 61d8658b ("scsi: qedf: Add QLogic FastLinQ offload FCoE driver framework.")
Acked-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>