gpiolib: cdev: replace locking wrappers for config_mutex with guards

After the adoption of guard(), the locking wrappers that hold the
config_mutex for linereq_set_values() and linereq_set_config() no
longer add value, so combine them into the functions they wrap.

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