ws2812: Fix first write making the first pixel green

The first epic_ws2812_write() call will set the first pixel to #008000. This is caused by the GPIO line being pulled down on mode-setting (epic_gpio_set_pin_mode). Wait before writing the values to reset the bus and thus properly set the pixels to the correct colors on first write.

The first LED will still flash green for a very short time. I don't know if there is a workaround for this ...

THIS IS UNTESTED!!! I do not have a neopixel with me right now, so I'd need someone else to confirm whether this works ...

If it works, this MR fixes #181 (closed).

Cc: @lilafisch