[SCSI] megaraid: remove a spurious IRQ enable

We took this lock with spin_lock() so we should unlock it with
spin_unlock() instead of spin_unlock_irq().  This was introduced in
f2c8dc402b "[SCSI] megaraid_mbox: remove scsi_assign_lock usage".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>