net: phy: Pass mdix ethtool setting through to phy driver

Pass the mdix setting from ethtool down to the phy driver, to allow
driver specific implementations of manually setting the polarity.

Signed-off-by: David Thomson <david.thomson@alliedtelesis.co.nz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>