net: dsa: mv88e6xxx: enable SerDes RX stats for Topaz

Commit 0df95287 ("mv88e6xxx: Add serdes Rx statistics") added
support for RX statistics on SerDes ports for Peridot.

This same implementation is also valid for Topaz, but was not enabled
at the time.

We need to use the generic .serdes_get_lane() method instead of the
Peridot specific one in the stats methods so that on Topaz the proper
one is used.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 0df95287 ("mv88e6xxx: Add serdes Rx statistics")
Signed-off-by: David S. Miller <davem@davemloft.net>