scsi: mpt3sas: Allow processing of events during driver unload.

Events were not processed during driver unload, hence unloading of
driver doesn't complete when drives are disconnected while unloading of
driver.  So don't block events in ISR path, i,e., remove the flag
ioc->remove_host so that events are getting processed during driver
unload.  Thus allowing driver unload to complete by processing drive
removal events during driver unload.

Signed-off-by: Chaitra P B <chaitra.basappa@broadcom.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>