netpoll: Should handle ETH_P_ARP other than ETH_P_IP in netpoll_neigh_reply

The received ARP request type in the Ethernet packet head is ETH_P_ARP other than ETH_P_IP.

[ Bug introduced by commit b7394d24
  ("netpoll: prepare for ipv6") ]

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>