net: mscc: ocelot: correctly report the timestamping RX filters in ethtool

The driver doesn't support RX timestamping for non-PTP packets, but it
declares that it does. Restrict the reported RX filters to PTP v2 over
L2 and over L4.

Fixes: 4e3b0468

 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>