gpiolib: add missed break statement

It's no difference in the functionality, but after the change the code
is less error prone to various checkers.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>

drivers/gpio/gpiolib.c

@@ -2107,6 +2107,7 @@ static int gpio_set_config(struct gpio_desc *desc, enum pin_config_param mode)
 
 	default:
 		arg = 0;
+		break;
 	}
 
 	config = PIN_CONF_PACKED(mode, arg);