power_supply: 88pm860x_charger: do not pass NULL to power_supply_put

In the case where power_supply_get_by_name returns NULL the current
error return path calls power_supply_put with a NULL psy which will
cause a null pointer dereference.  Avoid this with an immediate
return.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>