Some legacy RGMII phys don't have in band signaling for the
speed information. so set the RGMII MAC mode according to
the speed got from PHY.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Reported-by: John Traill <john.traill@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>