hpet config mask fix

I discovered a bug in the hpet code that caused Windows to boot without
hpet. The config mask I was using was preventing the guest from placing
the hpet into 32 bit mode.

(Beth Kon)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6357 c046a42c-6fe2-441c-8c8c-71466251a162
