staging: greybus: Use gpio_is_valid()

Replace the manual validity checks for the GPIO with the
gpio_is_valid().

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>