backlight: qcom-wled: Use sink_addr for sync toggle

WLED3_SINK_REG_SYNC is, as the name implies, a sink register offset.
Therefore, use the sink address as base instead of the ctrl address.

This fixes the sync toggle on wled4, which can be observed by the fact
that adjusting brightness now works.

It has no effect on wled3 because sink and ctrl base addresses are the
same.  This allows adjusting the brightness without having to disable
then reenable the module.

Signed-off-by: Obeida Shamoun <oshmoun100@googlemail.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Kiran Gunda <kgunda@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>