lan9303_mdio_write()/_read() must multiply register number by 4 to get
offset.

Added some commments to the register definitions.
Signed-off-by: NEgil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Reviewed-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>