net: dsa: mt7530: move pkt size and rx err MIB counter to rmon stats API

Drop custom handling of packet size and RX error MIB counter and handle
them in the standard .get_rmon_stats API

The MIB entry are dropped from the custom MIB table and converted to
a define providing only the MIB offset.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Link: https://patch.msgid.link/20250410163022.3695-3-ansuelsmth@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>