[NETROM]: statistics fix

Calling an incoming NET/ROM-encapsulated IP packet an error if the
interface isn't up is probably a bit over the top, so count it as
dropped instead of an error.

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