[PATCH] hpet: use read_timer_tsc only when CPU has TSC

Only use read_timer_tsc only when CPU has TSC.  Thanks to Andrea for
pointing this out.  Should not be issue on any platforms as all recent
systems that has HPET also has CPUs that supports TSC.  The patch is still
required for correctness.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>