n_gsm: use goto-failpaths in gsm_init

Use the classic failpath handling using gotos in gsm_init. That way,
tty_unregister_ldisc needs not be repeated on two places.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-16-jslaby@suse.cz


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