caif-hsi: fix possible deadlock in cfhsi_exit_module()

cfhsi_exit_module() calls unregister_netdev() under rtnl_lock().
but unregister_netdev() internally calls rtnl_lock().
So deadlock would occur.

Fixes: c4125400

 ("caif-hsi: Add rtnl support")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>