greybus: arche-apb: Replace gpio_is_valid() with gpio < 0 checks

There can be no invalid values in the DTS. The actual pin numbers are
assigned by gpiolib when the gpio controller is registered.

And so a simple 'gpio < 0' is enough instead of gpio_is_valid() which
also checks for 'gpio < ARCH_NR_GPIOS'.

This will make the usage of of_get_named_gpio() similar with how it is
done in arche-platform driver.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>