net: gptp: Fix sync receipt timeout timer starting

Completely remove the last_sync_receipt_timeout time. It is not part of
the standard (see 802.1AS-2011, 10.2.11 for the complete list of
variables for this state machine). Additionally this extra variable was
never really initialized so the calculated duration made no sense.

Just start the timer based on the regular sync receipt timeout time
interval from the port data set.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>