backlight: qcom-spmi-wled: Fix auto calibration brightness

Currently, when auto calibration algorithm runs because of an
OVP fault triggered by string misconfiguration, it sets a low
brightness level (200) to find the incorrect string by enforcing
the output feedback on each string. However, this doesn't help
finding the open string always as the WLED module may not be
regulating at a brightness lower than hybrid dimming threshold,
which is 6.25%.

As per the hardware recommendation, increase the auto calibration
brightness level to 512 (12.5%) which corrects the string
detection. While at it, change the regmap API that is used to
configure strings so that the write always happens as intended.

Change-Id: Ic5c6f31bde66a4f8db59279495237abdef0bfff7
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>