powercap/intel_rapl: Propagate error code when registration fails

If rapl_package_register_powercap() fails in rapl_add_package() the
function happily returns 0.

Capture the error code and propagate it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>