sfc: Use generic MDIO functions and definitions

Make use of the newly-added generic MDIO clause 45 support and remove
redundant definitions.

Add an 'efx_' prefix to the remaining driver-specific MDIO functions
and remove arguments which are redundant with efx->mdio.prtad.

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