clocksource: check range

Check that the value being passed to parse_pmtmr() does not exceed the
limits of pmtmr_ioport.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>