apa102: fix end frame

end frame is used to supply clock pulses so that data goes to last
LED in the chain. Thus, it depends on the number of LEDs in the chain.
Previously, the number of ones sent into the end frame was
hard-coded and limited the usage of the driver to 64 LEDs in the
strip.

Signed-off-by: Cyril Fougeray <cyril.fougeray@toolsforhumanity.com>