power: pm8921-bms: fix division error

Arithmetic conversion rules states that a signed operand is converted
to unsigned if the other operand is unsigned.

The voltage across vsense is a signed int and it can be negative.
Dividing it by the battery sense resistor, an unsigned int,  was
resulting in a very large positive number instead of a negative number.

Fix it by casting the battery sense resistor to signed so that a
a signed division is performed.

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