scsi: qla2xxx: Fix crash on qla2x00_mailbox_command

This patch fixes a crash on qla2x00_mailbox_command caused when the driver
is on UNLOADING state and tries to call qla2x00_poll, which triggers a
NULL pointer dereference.

Signed-off-by: Rodrigo R. Galvao <rosattig@linux.vnet.ibm.com>
Signed-off-by: Mauro S. M. Rodrigues <maurosr@linux.vnet.ibm.com>
Acked-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>