net: dsa: sja1105: Empty the RX timestamping queue on PTP settings change

When disabling PTP timestamping, don't reset the switch with the new
static config until all existing PTP frames have been timestamped on the
RX path or dropped. There's nothing we can do with these afterwards.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>