power: supply: bq27xxx: support missing supplier device

power_supply_am_i_supplied() can return negative error
codes. In this case we should assume, that no charger
is connected and the battery should be marked as
DISCHARGING instead of NOT_CHARGING.

Reported-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>