gpio: of: remove unnecessary variable in of_get_gpio_hog()

The variable "desc" is only used for storing the return value at the
end of the function.  It is unneeded.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>