drivers/timer/hpet: Disable FSB interrupts

The front side bus interrupt delivery feature is a somewhat obscure
part of PC history (in some sense a presaging of MSI interrupts) that
we don't use.

But it's part of the spec, works on hardware, has precedence over the
"legacy" interrupt routing feature we do use, and can be legally
enabled by firmware.

Disable at init time.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>