Revert "tty:n_gsm.c: destroy port by tty_port_destroy()"

This reverts commit 7726fb53.

Jiri writes:
	On 24. 09. 19, 11:25, Xiaoming Ni wrote:
	> According to the comment of tty_port_destroy():
	>     When a port was initialized using tty_port_init, one has to destroy
	>     the port by tty_port_destroy();

	It continues with a part saying:
	   Either indirectly by using tty_port refcounting
	   (tty_port_put) or directly if refcounting is not used.

So this should be reverted.

Cc: Xiaoming Ni <nixiaoming@huawei.com>
Reported-by: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>