backlight: qcom-wled: Fix unsigned comparison to zero

Fixes coccicheck warning:
./drivers/video/backlight/qcom-wled.c:1104:5-15:
	WARNING: Unsigned expression compared with zero: string_len > 0

The unsigned variable string_len is assigned a return value from the call
to of_property_count_elems_of_size(), which may return negative error code.

Fixes: 775d2ffb

 ("backlight: qcom-wled: Restructure the driver for WLED3")
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Kiran Gunda <kgunda@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>