net: usb: asix: suspend embedded PHY if external is used

In case external PHY is used, we need to take care of embedded PHY.
Since there are no methods to disable this PHY from the MAC side and
keeping RMII reference clock, we need to suspend it.

This patch will reduce electrical noise (PHY is continuing to send FLPs)
and power consumption by 0,22W.

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