backlight: qcom-wled: Override default length with qcom,enabled-strings

backlight: qcom-wled: Override default length with qcom,enabled-strings

The length of qcom,enabled-strings as property array is enough to
determine the number of strings to be enabled, without needing to set
qcom,num-strings to override the default number of strings when less
than the default (which is also the maximum) is provided in DT.

This also introduces an extra warning when qcom,num-strings is set,
denoting that it is not necessary to set both anymore.  It is usually
more concise to set just qcom,num-length when a zero-based, contiguous
range of strings is needed (the majority of the cases), or to only set
qcom,enabled-strings when a specific set of indices is desired.

Fixes: 775d2ffb4af6 ("backlight: qcom-wled: Restructure the driver for WLED3")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211115203459.1634079-6-marijn.suijten@somainline.org