net: phy: execute genphy_loopback() per default on all PHYs

The generic loopback is really generic and is defined by the 802.3
standard, we should just mandate that drivers implement a custom
loopback if the generic one cannot work.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>