gpiolib: Unify access to the device properties

Some of the functions are using struct fwnode_handle, some struct device
pointer. In the GPIO library the firmware node of the GPIO device is the
same as GPIO node of the GPIO chip. Due to this fact we may use former
to access properties everywhere in the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Brian Masney <bmasney@redhat.com>
Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
[Bartosz: stick to the 80-char limit where it's not hurting readability]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>