power: supply: Fix typo in power_supply_check_supplies

It seems to be a typo, there is no actual BUG, but it's better to
fix it to avoid any possible BUG after we change the type of
supplied_from.

Signed-off-by: Schspa Shi <schspa@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>