UEC PHY: Speed up initial PHY neg.

Instead of always performing an autoneg, check if the PHY
already has a link and if it matches one of the requested
modes. Initially only 100MbFD is optimized this way.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>