Ammeded timing to suit new revision of ws2812c (#439)

Should also work with WS2813 (but untested)

Included comments on the timing calculations

libsrc/leddevice/LedDeviceWs2812SPI.h

@@ -37,5 +37,7 @@ private:
 
         const int SPI_BYTES_PER_COLOUR;
 
+	const int SPI_FRAME_END_LATCH_BYTES;
+
 	uint8_t bitpair_to_byte[4];
 };