greybus: arche-platform: fix incorrect gpio variable type

GPIO number obtained from of_get_named_gpio() should be signed to allow
error handling.

Testing Done:
Built & booted on EVT1.5

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>