[SCSI] qla4xxx: Fix failure of mbox 0x31

Issue:
While unloading driver MBOX 0x31 fail as DDB logout (MBOX 0x56)
operation is not completed.

Fix:
Wait for DDB Logout completion before MBOX 0x31

Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>