bonding: enable netpoll without checking link status

Only slaves that are up should transmit netpoll frames, so there is no
need to check to see if a slave is up before enabling netpoll on it.
This resolves a reported failure on active-backup bonds where a slave
interface is down when netpoll was enabled.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Tested-by: WANG Cong <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>