[NETROM]: NET/ROM has no ARP

ARP over NET/ROM does not exist so it's obviously not implemented on any
NET/ROM stack, so the NET/ROM interfaces really should default to IFF_NOARP.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>