loopback: Add hooks to track port latency offsets

loopback: Add hooks to track port latency offsets

The previous patch assumed constant port latency offsets. The offsets can
however be changed by the user, therefore these changes need to be tracked
as well. This patch adds the necessary hooks.

Also the print_msg argument was removed from update_minimum_latency() and
update_latency_boundaries() because the message should always be logged.