RT-Thread use signed value for its pin numbers, while using -1 for
PIN_NONE. So when using U8X8_PIN_NONE, RT-Thread will incorrectly
interpret the pin as positive integer (255).
This patch wraps rt_pin_xxx functions to detect this condition.
It does limit the selectable pin ranges to 0-255, however it should
be enough for now.
Signed-off-by: Yilin Sun <imi415@imi.moe>
