gpiolib: cdev: fully adopt guard() and scoped_guard()

Use guard() or scoped_guard() for critical sections rather than
discrete lock/unlock pairs.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>