backlight: qcom-spmi-wled: Update i_sink calculation for WLED flash

When Vph is low because of low battery voltage and high system
load, i_sink calculated can have a zero because of not fitting
any of the conditional blocks. In real case, WLED flash is
possible even at this lower battery voltage. Hence set i_sink
to max_fsc to begin with and update it to a different voltage
based on the calculations later.

For an unsupported configuration, no calculations are available
to limit i_sink. Hence, limit the current to half of max_fsc
if Vph is below 3.2 V as per the hardware recommendation.

Change-Id: I8dcce6aa1722314006d6b34f6389ce640b8a25c3
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>