power: pm8921-bms: use consistent temperature units

There is bug in the code where interpolate_pc is passed temperature in
DegC at one call site vs deciDegCelcius at other.

Use degC consistently.

Change-Id: I3e5e92e39adf904e62d4c5ffcb6d370c0655f7cf
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>