net: phy: Support set_loopback override

phy_read_status and various other PHY functions support PHY specific
overriding of driver functions by using a PHY specific pointer to the
PHY driver. Add support of PHY specific override to phy_loopback too.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>