net: mvpp2: Rename mvpp2_ethtool_counters to mvpp2_ethtool_mib_counters

Since we'll be adding support for other kind of internal counters, make
clear that the currently supported counters are the MIB counters.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>