hw/core/ptimer: fix timer zero period condition for freq > 1GHz

hw/core/ptimer: fix timer zero period condition for freq > 1GHz

The real period is zero when both period and period_frac are zero.
Check the method ptimer_set_freq, if freq is larger than 1000 MHz,
the period is zero, but the period_frac is not, in this case, the
ptimer will work but the current code incorrectly recognizes that
the ptimer is disabled.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2306
Signed-off-by: JianZhou Yue <JianZhou.Yue@verisilicon.com>
Message-id: 3DA024AEA8B57545AF1B3CAA37077D0FB75E82C8@SHASXM03.verisilicon.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>