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

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

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();

tty_port_init() is called in gsm_dlci_alloc()
so tty_port_destroy() needs to be called in gsm_dlci_free()

Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Link: https://lore.kernel.org/r/1569317156-45850-1-git-send-email-nixiaoming@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>