net: phy: genphy_loopback: add link speed configuration

In case of loopback, in most cases we need to disable autoneg support
and force some speed configuration. Otherwise, depending on currently
active auto negotiated link speed, the loopback may or may not work.

This patch was tested with following PHYs: TJA1102, KSZ8081, KSZ9031,
AT8035, AR9331.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>