forcedeth: remove unnecessary carrier status check

Since netif_carrier_on() will do nothing if device's
carrier is already on, so it's unnecessary to do
carrier status check.

It's the same for netif_carrier_off().

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>