leds: leds-qpnp-flash: add max current prediction

PMIC based flash LED cannot support full LED intensity when battery
level is low. When battery level is low and a user wants to use full
intensity, PMIC clamps LED current to avoid under voltage lock
out(UVLO). Therefore, before flash LED assigns brightness, it checks
for maximum allowed current at current battery level. If current
requested is larger than PMIC can provide, flash LED current will
be truncated.

Change-Id: I17f1ef27358c91c13e4f5f8df08761fc944e9345
Signed-off-by: Chun Zhang <chunz@codeaurora.org>