gpio: just plain warning when nonexisting gpio requested

When trying to export an nonexisting gpio ID, the kernel prints out a
big warning w/ stacktrace, sounding like a huge problem. In fact it's
a pretty normal situation, like file or device not found.

So, just print a more relaxed warning instead.

changes v2: drop defining pr_fmt()
Signed-off-by: Enrico Weigelt <info@metux.net>
Link: https://lore.kernel.org/r/20201202133754.32045-1-info@metux.netSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

 // SPDX-License-Identifier: GPL-2.0
+
 #include <linux/bitmap.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
@@ -119,7 +120,7 @@ struct gpio_desc *gpio_to_desc(unsigned gpio)
 	spin_unlock_irqrestore(&gpio_lock, flags);
 
 	if (!gpio_is_valid(gpio))
-		WARN(1, "invalid GPIO %d\n", gpio);
+		pr_warn("invalid GPIO %d\n", gpio);
 
 	return NULL;
 }