net: airoha: Add get_link ethtool callback

Set get_link ethtool callback to ethtool_op_get_link routine.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251013-airoha-ethtool-improvements-v1-2-fdd1c6fc9be1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>