gpiolib: add desc validation to gpiod_set_transitory()

The gpiod_set_transitory() function is publicly exported, and
it is expected from it to be ready for usage with optional GPIOs
on consumer's side.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpiolib.c

@@ -2526,6 +2526,7 @@ int gpiod_set_transitory(struct gpio_desc *desc, bool transitory)
 	int gpio;
 	int rc;
 
+	VALIDATE_DESC(desc);
 	/*
 	 * Handle FLAG_TRANSITORY first, enabling queries to gpiolib for
 	 * persistence state.