greybus: power_supply: fix lock imbalance in init error path

Make sure to release the supplies_lock before returning on errors in
gb_power_supplies_setup().

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>