power: supply: bq27xxx: fix polarity of current_now

current_now has to be negative during discharging and positive during
charging, the behavior seen is the other way round.

Tested on GTA04 with Openmoko battery.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>