ARM: fix forced-HZ values

Olof Johansson reports that the tests against HZ_FIXED seem
non-functional.  Fix this by using '0' as a sentinel for "not
specified" and test against that instead.

Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>