qpnp-fg-gen3: fix the error in showing battery temperature

Battery temperature can take negative values. FG driver reads it
as an unsigned value and hence the negative reading is treated as
a huge integer value. Fix it.

CRs-Fixed: 1081146
Change-Id: I63e8863efb91af891dbcbfc070dfdcd833ea3ad4
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>