PTP RX timestamping should be enabled when the user requests it, not by
default. If it is enabled by default, it can be problematic when the
ocelot driver is a DSA master, and it sidesteps what DSA tries to avoid
through __dsa_master_hwtstamp_validate().

Additionally, after the change which made ocelot trap PTP packets only
to the CPU at ocelot_hwtstamp_set() time, it is no longer even true that
RX timestamping is enabled by default, because until ocelot_hwtstamp_set()
is called, the PTP traps are actually not set up. So the rx_filter field
of ocelot->hwtstamp_config reflects an incorrect reality.

Fixes: 96ca08c0 ("net: mscc: ocelot: set up traps for PTP packets")
Fixes: 4e3b0468 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>