gpio: Use unsigned int for of_gpio_n_cells

The cell count for GPIO specifiers can never be negative, so make the
field unsigned.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -193,7 +193,7 @@ struct gpio_chip {
 	 *
 	 * Number of cells used to form the GPIO specifier.
 	 */
-	int of_gpio_n_cells;
+	unsigned int of_gpio_n_cells;
 
 	/**
 	 * @of_xlate: