hvm/hpet: Correctly limit period to a maximum.

hvm/hpet: Correctly limit period to a maximum.

In the code section after the comment:

    /*
     * Clamp period to reasonable min/max values:
     *  - minimum is 100us, same as timers controlled by vpt.c
     *  - maximum is to prevent overflow in time_after() calculations
     */

The current maximum limit actually allows "bad" values like 0 and 1.
This is because it uses a mask not a maximum.

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>