ftgmac100: Cleanup speed/duplex tracking and fix duplex config

Keep track of both the current speed and duplex settings
instead of only speed and properly apply the duplex setting
to the HW.

This reworks the adjust_link() function to also avoid trying
to reconfigure the HW when there is no link and to display
the link state to the user.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>