gpiolib-acpi: remove unused variable 'err', cleans up build warning

Variable err is defined but never used. Remove it. Cleans up warning:

warning: unused variable ‘err’ [-Wunused-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>