gpio: constify the data parameter to gpiochip_find()

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

diff --git a/drivers/of/gpio.c b/drivers/of/gpio.c
index e034b38..bba8121 100644
--- a/drivers/of/gpio.c
+++ b/drivers/of/gpio.c
@@ -229,7 +229,7 @@
 }
 
 /* Private function for resolving node pointer to gpio_chip */
-static int of_gpiochip_is_match(struct gpio_chip *chip, void *data)
+static int of_gpiochip_is_match(struct gpio_chip *chip, const void *data)
 {
 	return chip->of_node == data;
 }