[SCSI] isci: kill isci_port->status

It only tracks whether the port is stopping in order to gate new devices
being discovered while the port is stopping.  However, since the check
and subsequent handling is unlocked there is nothing to stop the port
from going down immediately after the check.

Driver is already prepared to handle devices arriving on stale ports,
and those will be cleaned up by an eventual ->lldd_dev_gone()
notification.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>