scsi: libfc: do not flood console with messages 'libfc: queue full ...'

When the FCoE sending side becomes congested libfc tries to reduce the
queue depth on the host; however due to the built-in lag before
attempting to ramp down the queue depth _again_ the message log is
flooded with the following message:

	libfc: queue full, reducing can_queue to 512

With this patch the message is printed only once (ie when it's
actually changed).

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