n_gsm: switch dead to bool

Both gsm_dlci->dead and gsm_mux->dead are used as bools, so treat them
as such.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-7-jslaby@suse.cz


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