net: sh_eth: fix cannot work half-duplex mode

When link was down, the bit of DM in ECMR was always set.
So, we could not use half-duplex mode on the controller.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>