gpiolib: remove redundant initialization of pointer desc

The initialized value stored in pointer desc is never read as it
is updated in the first executable statement in the function.
This is therefore redundant and can be removed.

Cleans up clang warning:
drivers/gpio/gpiolib.c:3710:20: warning: Value stored to 'desc'
during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>