driver/net/fec: support fixed speed connection

If MAC is directly connected to another MAC (like a switch for example)
we don't need to probe for a phy, autoneogation and so on. We simply
have to setup speed.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>