Added support for Diagnostics MBOX command via BSG Vendor HST_VENDOR
interface.  This command provides various tests for validating hardware
functionality.
Signed-off-by: NVikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: NMike Christie <michaelc@cs.wisc.edu>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>