netif: Don't return error if pass NULL netif_input_fn to netif_add

The new NULL checking against netif_input_fn make ppp stop working
because in ppp_new() it calls netif_add with NULL netif_input_fn.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

src/core/netif.c

@@ -294,7 +294,6 @@ netif_add(struct netif *netif,
 
   LWIP_ERROR("netif_add: invalid netif", netif != NULL, return NULL);
   LWIP_ERROR("netif_add: No init function given", init != NULL, return NULL);
-  LWIP_ERROR("netif_add: No input function given", input != NULL, return NULL);
 
 #if LWIP_IPV4
   if (ipaddr == NULL) {