power: charger-manager: clarify num_properties starting value

Initialize num_properties with length of the copied array instead
of relying on previously memcpy'd value. This makes it clear how
the array and the counter are related.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>