n_gsm: switch constipated to bool

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

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-9-jslaby@suse.cz


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