netpoll: use netpoll_rx_on() in netpoll_rx()

The logic of the code is same, just call netpoll_rx_on().

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>