backlight: qcom-spmi-wled: Calculate maximum available current for flash

For WLED flash or preflash, add support to calculate the maximum
available current based on the battery parameters, number of
strings and number of LEDs per string.

Now that the maximum available current is predicted for WLED
flash, change the default full scale current for WLED flash and
preflash to 50 mA.

Change-Id: I7a28060798ce796b7dc72d8ba4db3f195316b950
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>