x86: fix bootup crash in native_read_tsc()

fix bootup crash in native_read_tsc() that was reported on an Athlon-XP
and bisected. The correct feature boundary for X86_FEATURE_MFENCE_RDTSC
is not XMM but XMM2.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>