tipc: make bearer set up in module insertion stage

Accidentally a side effect is involved by commit 6e967adf(tipc:
relocate common functions from media to bearer). Now tipc stack
handler of receiving packets from netdevices as well as netdevice
notification handler are registered when bearer is enabled rather
than tipc module initialization stage, but the two handlers are
both unregistered in tipc module exit phase. If tipc module is
inserted and then immediately removed, the following warning
message will appear:

"dev_remove_pack: ffffffffa0380940 not found"

This is because in module insertion stage tipc stack packet handler
is not registered at all, but in module exit phase dev_remove_pack()
needs to remove it. Of course, dev_remove_pack() cannot find tipc
protocol handler from the kernel protocol handler list so that the
warning message is printed out.

But if registering the two handlers is adjusted from enabling bearer
phase into inserting module stage, the warning message will be
eliminated. Due to this change, tipc_core_start_net() and
tipc_core_stop_net() can be deleted as well.

Reported-by: Wang Weidong <wangweidong1@huawei.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>