gpiolib: cdev: compile out HTE unless CONFIG_HTE selected

The majority of builds do not include HTE, so compile out hte
functionality unless CONFIG_HTE is selected.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>