scsi: mpt3sas: Remove usage of device_busy counter

Remove usage of device_busy counter from driver. Instead of device_busy
counter now driver uses 'nr_active' counter of request_queue to get the
number of inflight request for a LUN.

Link: https://lore.kernel.org/r/20191226111333.26131-10-sreekanth.reddy@broadcom.com


Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>