[SCSI] zfcp: Assign scheduled work to driver queue

The port_scan work was scheduled to the work_queue provided by the
kernel. This resulted on SMP systems to a likely situation that more
than one scan_work were processed in parallel. This is not required
and openes the possibility of race conditions between the removal of
invalid ports and the enqueue of just scanned ports.  This patch
synchronizes the scan_work tasks by scheduling them to adapter local
work_queue.

Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>