xen-acpi-processor: Don't display errors when we get -ENOSYS

which is a perfectly legal error. This can be triggered if the
user has booted Xen with the no-cpuidle parameter.

Reported-by-and-Tested-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>