x86: fix cpu MHz reporting for AMD family 0x11

Fix cpu MHz reporting for AMD family 0x11 when powernow-k8 is
disabled.

Just adhere to the CONSTANT_TSC feature bit for AMD CPUs when deciding
whether cpu_khz needs calibration. The additional check for CPU family
is not needed and prevents calibration for future CPUs.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>