tsnep: Select speed for loopback

tsnep: Select speed for loopback

Use 100 Mbps only if the PHY is configured to this speed. Otherwise use
always the maximum speed of 1000 Mbps.

Also remove explicit setting of carrier on and link mode after loopback.
This is not needed anymore, because phy_loopback() with selected speed
signals the link and the speed to the MAC.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250312203010.47429-6-gerhard@engleder-embedded.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>