ws2812: make AVR support more robust

* Merge AVR support for Digispark and non-Digispark
* Fix the error "inline assembly requires more registers than available"
* Use a single file for all AVR targets, in a similar style as the
  Xtensa support.

ws2812/ws2812_xtensa.go

@@ -4,13 +4,10 @@ package ws2812
 
 import (
 	"device"
-	"errors"
 	"machine"
 	"unsafe"
 )
 
-var errUnknownClockSpeed = errors.New("ws2812: unknown CPU clock speed")
-
 func (d Device) WriteByte(c byte) error {
 	portSet, maskSet := d.Pin.PortMaskSet()
 	portClear, maskClear := d.Pin.PortMaskClear()