scsi: mpt3sas: simplify task management functions

No functional change. Code optimization.

One can simply check 'target_busy' or 'device_busy' when figuring out if
there are outstanding commands; no need to painstakingly count them by
hand.

[mkp: tweaked patch description]

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>