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>

net/6lowpan/iphc.c

@@ -811,8 +811,8 @@ static u8 lowpan_compress_ctx_addr(u8 **hc_ptr, const struct in6_addr *ipaddr,
 		goto out;
 	}
 
-	WARN_ON_ONCE("context found but no address mode matched\n");
-	return -EINVAL;
+	WARN_ONCE(1, "context found but no address mode matched\n");
+	return LOWPAN_IPHC_DAM_00;
 out:
 
 	if (sam)