bq27x00: Fix CURRENT_NOW property

According to the bq27000 datasheet the current should be calculated by
the following formula:
    current = AI * 3570 / 20

This patch adjust the drivers code accordingly.

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