power: pm8921-bms: change r_sense unit from mOhm to uOhm

Change the unit of r_sense from Milli Ohms to Micro Ohms in order to
increase the precision of ccadc and current readings.

Change-Id: Ic3f2c4e5bafd3a1556412005564fc0b549e6813c
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>