timers: don't #error on higher HZ values

For some crazy reason (trying to work around hw problem in i810) I wanted
to use HZ around 4000.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>