net: dsa: add plumbing for changing and getting MAC merge layer state

The DSA core is in charge of the ethtool_ops of the net devices
associated with switch ports, so in case a hardware driver supports the
MAC merge layer, DSA must pass the callbacks through to the driver.
Add support for precisely that.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>