gpio: gpiolib-of: Use PTR_ERR_OR_ZERO

PTR_RET is deprecated. Use PTR_ERR_OR_ZERO instead. While at it
also include missing err.h header.

Signed-off-by: Sachin Kamat <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpiolib-of.c

···
  */
 
 #include <linux/device.h>
+#include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/module.h>
 #include <linux/io.h>
···
 
 	of_node_put(gg_data.gpiospec.np);
 	pr_debug("%s exited with status %d\n", __func__,
-		 PTR_RET(gg_data.out_gpio));
+		 PTR_ERR_OR_ZERO(gg_data.out_gpio));
 	return gg_data.out_gpio;
 }
 EXPORT_SYMBOL(of_get_named_gpiod_flags);