net: dsa: lan9303: Multiply by 4 to get MDIO register

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

Added some commments to the register definitions.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>