gpio: remove the RW semaphore from the GPIO device

With all accesses to gdev->chip being protected with SRCU, we can now
remove the RW-semaphore specific to the character device which
fulfilled the same role up to this point.

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>