hw/timer/hpet: Reorganize register decoding

hw/timer/hpet: Reorganize register decoding

For Rust HPET, since the commit 519088b7cf6d ("rust: hpet: decode HPET
registers into enums"), it decodes register address by checking if the
register belongs to global register space. And for C HPET, it checks
timer register space first.

While both approaches are fine, it's best to be as consistent as
possible.

Synchronize changes from the rust side to C side.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250520152750.2542612-2-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>