HID: wacom: Fix error path of power-supply initialization

power_supply_unregister() must not be called if power_supply_register() failed.
The wdata->psy.dev pointer may point to invalid memory after a failed
power_supply_register() and hence wacom_remove() will fail while calling
power_supply_unregister().

This changes the wacom_probe function to fail if it cannot register the
power_supply devices. If we would want to keep the previous behaviour we had to
keep some flag about the power_supply state and check it on wacom_remove, but
this seems inappropriate here. Hence, we simply fail, too, if
power_supply_register fails.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>