net: phy: icplus: add MDI/MDIX support for IP101A/G

Implement the operations to set desired mode and retrieve the current
mode.

This feature was tested with an IP101G.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>