rust: hpet: fix decoding of timer registers

Due to a missing "& 0x18", timer registers are not decoded correctly.
This breaks the tests/functional/test_x86_64_tuxrun.py functional
test.

Fixes: 519088b7cf6 ("rust: hpet: decode HPET registers into enums", 2025-03-06)
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/timer/hpet/src/hpet.rs

@@ -776,7 +776,7 @@ fn decode(&self, mut addr: hwaddr, size: u32) -> HPETAddrDecode {
             let timer_id: usize = ((addr - 0x100) / 0x20) as usize;
             if timer_id <= self.num_timers.get() {
                 // TODO: Add trace point - trace_hpet_ram_[read|write]_timer_id(timer_id)
-                TimerRegister::try_from(addr)
+                TimerRegister::try_from(addr & 0x18)
                     .map(|reg| HPETRegister::Timer(&self.timers[timer_id], reg))
             } else {
                 // TODO: Add trace point -  trace_hpet_timer_id_out_of_range(timer_id)