net: dsa: lan9303: MDIO access phy registers directly

Indirect access (PMI) to phy register only work in I2C mode. In
MDIO mode phy registers must be accessed directly. Introduced
struct lan9303_phy_ops to handle the two modes.

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>