net: 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: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>