scsi: megaraid_sas: megasas_get_request_descriptor always return valid desc

No functional change. Code clean up. Removing error code which is not
valid scenario.  In megasas_get_request_descriptor we can remove the
error handling which is not required.  With fusion controllers, if there
is a valid message frame available, we are guaranteed to get a
corresponding request descriptor.

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>