xen/acpi: off by one in read_acpi_id()

If acpi_id is == nr_acpi_bits, then we access one element beyond the end
of the acpi_psd[] array or we set one bit beyond the end of the bit map
when we do __set_bit(acpi_id, acpi_id_present);

Fixes: 59a56802

 ("xen/acpi-processor: C and P-state driver that uploads said data to hypervisor.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>