net: ipip: fix wrong address family in init error path

In case of error with MPLS support the code is misusing AF_INET
instead of AF_MPLS.

Fixes: 1b69e7e6 ("ipip: support MPLS over IPv4")
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>