emac: add missing support of 10mbit in emac/rgmii

In chips of emac/rgmii b'000' for 0/1 channel isn't suitable which
resulted in non working network interface in this mode.

Signed-off-by: Ivan Mikhaylov <ivan@ru.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>