power: supply: adp5061: show unknown capacity_level as text

adp5061_get_battery_status() only defined show chg_status <= 4, others will
be show as '-1731902199' from /sys/class/power_supply/xx/capacity_level.
switch to show them as 'Unknown'.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>