port: prime sync rx timeout when entering PS_SLAVE in P2P mode

In P2P mode if an invalid sync/fup pair is immediately followed by an announce
msg for a better foreign clock and there are no valid sync/fup pairs afterwards
then the sync rx timeout never fires and PS_MASTER is never entered.

Signed-off-by: Delio Brignoli <dbrignoli@audioscience.com>