PowerCap: Fix an error code in powercap_register_zone()

PowerCap: Fix an error code in powercap_register_zone()

In the current code we accidentally return the successful result from
idr_alloc() instead of a negative error pointer.  The caller is looking
for an error pointer and so it treats the returned value as a valid
pointer.

This one might be a bit serious because if it lets people get around the
kernel's protection for remapping NULL.  I'm not sure.

Fixes: 75d2364ea0ca (PowerCap: Add class driver)
Signed-off-by: Dan Carpenter <[email protected]>
Reviewed-by: Srinivas Pandruvada <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>