mdio: Add mdio45_ethtool_spauseparam_an()

This implements the ETHTOOL_SPAUSEPARAM operation for MDIO (clause 45)
PHYs with auto-negotiation MMDs.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>