gpio: of: Fix NPE from OF flags

Some calls to of_get_named_gpio() calls sets the flags
argument to NULL because they are not interested in the
flags. This caused a null pointer exception since we were
unconditionally using these flags. Fix it.

Fixes: 6a537d48461d ("gpio: of: Support regulator nonstandard GPIO properties")
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib-of.c

@@ -129,7 +129,8 @@ struct gpio_desc *of_get_named_gpiod_flags(struct device_node *np,
 	if (IS_ERR(desc))
 		goto out;
 
-	of_gpio_flags_quirks(np, flags);
+	if (flags)
+		of_gpio_flags_quirks(np, flags);
 
 	pr_debug("%s: parsed '%s' property of node '%pOF[%d]' - status (%d)\n",
 		 __func__, propname, np, index,