We should return -EINVAL (instead of 0) if get_cpu_device() fails.

Fixes: 158c998e (ACPI / CPPC: add sysfs support to compute delivered performance)
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>