power: supply: bq27xxx: fix sign of current_now for newer ICs

Commit cd060b4d ("power: supply: bq27xxx: fix polarity of current_now")
changed the sign of current_now for all bq27xxx variants, but on BQ28Z610
I'm now seeing negated values *with* that patch.

The GTA04/Openmoko device that was used for testing uses a BQ27000 or
BQ27010 IC, so I assume only the BQ27XXX_O_ZERO code path was incorrect.
Revert the behaviour for newer ICs.

Fixes: cd060b4d

 "power: supply: bq27xxx: fix polarity of current_now"
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>