net: cdc-ncm: Move spin_lock_bh() to spin_lock()

It is unnecessary to call spin_lock_bh() for you are already in a tasklet.

Signed-off-by: Yunbo Yu <yuyunbo519@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>