[SCSI] qla4xxx: Added support for Diagnostics MBOX command

Added support for Diagnostics MBOX command via BSG Vendor HST_VENDOR
interface.  This command provides various tests for validating hardware
functionality.

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>