mISDN: Use DEFINE_SPINLOCK() for spinlock

spinlock can be initialized automatically with DEFINE_SPINLOCK()
rather than explicitly calling spin_lock_init().

Changelog:
From v1:
1. fix the mistake reported by kernel test robot.

Signed-off-by: Shixin Liu <liushixin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>