x86 driver: fix typo in TDP override enabling

When enabling turbo, we need to set both the TDC and TDP bits.  IIRC
only the TDC one actually matters, but fix it up anyway since the
current code is confusing.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Matthew Garrett <mjg@redhat.com>