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>