gpio: cdev: don't access gdev->chip if it's not needed

The variable holding the number of GPIO lines is duplicated in GPIO
device so read it instead of unnecessarily dereferencing the chip
pointer.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>