net: fix a build failure when !CONFIG_PROC_FS

When !CONFIG_PROC_FS dev_mcast_init() is not defined,
actually we can just merge dev_mcast_init() into
dev_proc_init().

Reported-by: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>