kconfig: gconf: show checkbox for choice correctly

Currently, bool choices have a checkbox, but tristate choices do not.
It is opposite.

Bool choices should not have a checkbox, as they are fixed to 'y' since
commit 6a121588 ("kconfig: remove 'optional' property support").
Tristate choices, however, should have a checkbox to allow users to
toggle the value.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>