net: ethernet: fs_enet: drop the .adjust_link custom fs_ops

There's no in-tree user for the fs_ops .adjust_link() function, so we
can always use the generic one in fe_enet-main.

Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>