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