scsi: libfc: Replace ->rport_flush_queue callback with function call

The ->rport_flush_queue callback only ever had a single
implementation, so we can as well call it directly and
drop the callback.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>