leds: leds-qpnp-flash: update flash LED register setting

In old PMIC document, there are errors regarding thermal derate rate and
update settings of ramp step, and phone voltage droop setting to
achieve better performance.

Change-Id: I35d3e2d94e6ac43c037a9ccc6302d7b93655f211
Signed-off-by: Chun Zhang <chunz@codeaurora.org>