rust: hpet: fix decoding of timer registers

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>