net/macb: Create gem_ethtool_ops for new statistics functions

10/100 MACB does not have the same statistics possibilities as GEM. Separate
macb_ethtool_ops to make a new GEM-specific struct with the new statistics
functions included.

Signed-off-by: Xander Huff <xander.huff@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>