bq27x00: Improve temperature property precession

This patch improves the precession of the temperature property of the
bq27x00 driver.
By dividing before multiplying the current code effectively cuts of the
last decimal digit. This patch fixes it by multiplying before dividing.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>