[NETPOLL]: netpoll_poll() cleanup

Restructure code slightly to improve readability:
  * dereference device once
  * change obvious while() loop
  * let poll_napi() handle null list itself

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>