dt-bindings: leds: bd2606mvv: Fix maximum register value

Since the chip can drive up to 6 lines, the maximum
register number should be only 5, not 6.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20230704202929.91962-1-marex@denx.de


Signed-off-by: Lee Jones <lee@kernel.org>