net: ethernet: Only try ARP for IP packets

The work in Ethernet send in commit 2f10d7d8
("net: ethernet: Set the ptype by the caller in send")
introduced a bug because we could try to do ARP resolving
for ARP packets too. This is clearly a wrong thing to do.
So before trying to do ARP resolving, make sure the the
packet type is IP packet.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>