hpet: clean up warning

Fix the below compile warnings due to recent HPET MSI changes

arch/x86/kernel/hpet.c:48: warning: 'hpet_devs' defined but not used
arch/x86/kernel/hpet.c:50: warning: 'per_cpu__cpu_hpet_dev' defined but not used

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>