net: phy: expose PHY device interface mode

Expose the PHY device interface mode through sysfs since this is an
useful piece of information for knowing how the attached networking
device will have configured its transmit/receive path.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>