clocksource: make CLOCKSOURCE_MASK bullet-proof

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>