intel_idle: correct BXT support

Commit 5dcef694 ("intel_idle: add BXT support") added an 8-element
lookup array with just a 2-bit value used for lookups. As per the SDM
that bit field is really 3 bits wide. While this is supposedly benign
here, future re-use of the code for other CPUs might expose the issue.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>