Check whether pp_instance port link is up or down. Force a new
initialization in case link brings up. Do not call state machine for
pp_instances whose link is down.

This patch is inspired to how ptp-noposix (commit 2e46712e59) handles link
status.

Still missing (and thus declared with a FIXME) the reset of slave structures.
On the single link, this lack does not cause issues. Anyway, must be fixed
Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>