scsi: hpsa: separate monitor events from rescan worker

scsi: hpsa: separate monitor events from rescan worker

create new worker thread to monitor controller events
 - both the rescan and event monitor workers can cause a rescan to occur
   however for multipath we have found that we need to respond faster
   than the normal scheduled rescan interval for path fail-overs.
 - getting controller events only involves reading a register, but the
   rescan worker can obtain an updated LUN list when there is a PTRAID
   device present.
 - move common code to a separate function.
advantages:
 - detect controller events more frequently.
 - leave rescan thread interval at 30 seconds.

Reviewed-by: Scott Benesh <scott.benesh@microsemi.com>
Reviewed-by: Scott Teel <scott.teel@microsemi.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microsemi.com>
Signed-off-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>