At the WS2812 driver level, a 400ns value was used for T0H (time high to
send a 0 bit) but LED specification says it should be 350ns +- 150ns.
Due to loop overhead the 400ns value could lead to T0H close to 500ns
which is too close from the limit value and gave glitches (bad data to
pixels) in some cases. This patch makes the calculated T0H value 350ns.
Interrupts during neopixel_write causes timing problems and therefore
wrong light patterns. Switching off IRQs should help to keep the strict
timing schedule.
