net: ipv6: use static inlines where it makes sense for stubs

If we're disabling certain features and instead providing function stubs,
it doesn't make sense to use macros for those routines that return values
that the user may check unless we want to provide them with vague errors
like:

src/coap-server.c:101:55: error: expected expression before ';' token
  101 |  ret = net_ipv6_mld_join(iface, &mcast_addr.sin6_addr);

When a function we're mocking up returns a value, use a static inline
stub.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>