power: supply: charger-manager: fix incorrect check on charging_duration_ms

Currently the duration check on the discharging duration setting is
checking the charging duration rather than the discharging duration
due to a cut-n-paste coding error. Fix this by checking the value
desc->charging_max_duration_ms.

Addresses-Coverity: ("Copy-paste-error")
Fixes: 8fcfe088 ("charger-manager: Support limit of maximum possible")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>