scsi: qla2xxx: Add lock protection around host lookup

Host lookup via btree is currently protected by the hardware_lock.
Add hardware_lock when modifying btree to store host pointer.

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>