bq27x00: Give more specific reports on battery status

The current code only reports whether the battery is charging or
discharging. But the battery also reports whether it is fully charged,
furthermore by look at if the battery is supplied we can tell whether it
is discharging or not charging.

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