net: macb: add support for C45 MDIO read/write

This patch modify MDIO read/write functions to support
communication with C45 PHY.

Signed-off-by: Milind Parab <mparab@cadence.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>