scsi: mpt3sas: simplify _wait_for_commands_to_complete()

Use 'host_busy' instead of counting outstanding commands by hand.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>