mdio: Export mdio.h to userland

The ID packing definitions are needed by userland and the register
definitions may also be useful there.

Do not export mdio_phy_id_{is_c45,prtad,devad}() as the use of bool is
problematic and it's not that useful to export only a subset of these.

Do not export MDIO_SUPPORTS_{C22,C45} directly; these flags are only
exposed to userland through struct ethtool_cmd so they should be
defined alongside that with appropriate names.

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