net: phy: Allow loopback speed selection for PHY drivers

net: phy: Allow loopback speed selection for PHY drivers

PHY drivers support loopback mode, but it is not possible to select the
speed of the loopback mode. The speed is chosen by the set_loopback()
operation of the PHY driver. Same is valid for genphy_loopback().

There are PHYs that support loopback with different speeds. Extend
set_loopback() to make loopback speed selection possible.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250312203010.47429-2-gerhard@engleder-embedded.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>