IB/srpt: Update sport->port_guid with each port refresh

If port_guid is set with the default subnet_prefix, then we get a change
event and run a port refresh, we don't update the port_guid.  As a
result, attempts to create a target device that uses the new
subnet_prefix in the wwn will fail to find a match and be rejected by
the ib_srpt driver.  This makes it impossible to configure a port if it
was initialized with a default subnet_prefix and later changed to any
non-default subnet-prefix.  Updating the port refresh task to always
update the wwn based upon the current subnext_prefix solves this
problem.

Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: nab@linux-iscsi.org
Signed-off-by: Doug Ledford <dledford@redhat.com>