power: smb135x-charger: reverse boost fixes

On some devices the battery is powered via external source which
could raise its voltage above the float voltage. smb135x chips go
in to unintentional reverse boost in such a situation and the
workaround is to disable float voltage compensation (note that
the battery will appear hot/cold when powered via external source).

While at it change the names of few config registers to begin with
CFG instead of CHG.

Change-Id: I32c4662720ed81599fd9e8be6e725fb9fe943600
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>