net: ethernet: fs_enet: phylink conversion

fs_enet is a quite old but still used Ethernet driver found on some NXP
devices. It has support for 10/100 Mbps ethernet, with half and full
duplex. Some variants of it can use RMII, while other integrations are
MII-only.

Add phylink support, thus removing custom fixed-link hanldling.

This also allows removing some internal flags such as the use_rmii flag.

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