tsnep: Add link down PHY loopback support

tsnep: Add link down PHY loopback support

PHY loopback turns off link state change signalling. Therefore, the
loopback only works if the link is already up before the PHY loopback is
activated.

Ensure that PHY loopback works even if the link is not already up during
activation by calling netif_carrier_on() explicitly.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Link: https://lore.kernel.org/r/20240123200151.60848-1-gerhard@engleder-embedded.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>