scsi: qla4xxx: remove unnecessary condition check for dma_pool_destroy()

dma_pool_destroy() can handle NULL pointer correctly, so there is no
need to check NULL pointer before calling dma_pool_destroy().

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Acked-by: Manish Rangankar <Manish.Rangankar@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>