[SCSI] qla2xxx: Further generalization of SRB CTX infrastructure.

Prepare CTX infrastructure for additional asynchronous
executions, add generic done() operator, pull CMD definitions.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>