tty: n_gsm: use DEFINE_SPINLOCK() for spinlock

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

Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Guobin Huang <huangguobin4@huawei.com>
Link: https://lore.kernel.org/r/1617710163-48158-1-git-send-email-huangguobin4@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>