6lowpan: iphc: fix invalid case handling

This patch fixes the return value in a case which should never occur.
Instead returning "-EINVAL" we return LOWPAN_IPHC_DAM_00 which is
invalid on context based addresses. Also change the WARN_ON_ONCE to
WARN_ONCE which was suggested by Dan Carpenter.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>