gpiolib: Extract gpio_chip_get_value() wrapper

In couple of cases we are using the same code to wrap ->get() callback.
Extract that code into a helper for the sake of better maintenance.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>