power: pm8921-charger: Use min of therm and cool/warm current choice

Currently there are two entities that control the charge current,
the cool/warm zone and the thermal daemon. An issue was observed
where changes to cool/warm zone overwrites the charge current
set by the thermal daemon.

Fix this by selecting the min chg current for the above cases.

Change-Id: I644ce0aff86c3eea403ece220825bba776c9feb5
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>