net: phy: Support speed selection for PHY loopback

phy_loopback() leaves it to the PHY driver to select the speed of the
loopback mode. Thus, the speed of the loopback mode depends on the PHY
driver in use.

Add support for speed selection to phy_loopback() to enable loopback
with defined speeds. Ensure that link up is signaled if speed changes
as speed is not allowed to change during link up. Link down and up is
necessary for a new speed.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Link: https://patch.msgid.link/20250312203010.47429-3-gerhard@engleder-embedded.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>