mdio: Expose pause frame advertising flags to ethtool

In mdio45_ethtool_gset_npage() and mdio45_ethtool_gset(), check MDIO
pause frame advertising flags and set the corresponding ethtool flags.

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