scsi: qla2xxx: Remove unnecessary null check

A null check before dma_pool_destroy is redundant, so remove it. This is
detected by coccinelle.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>