hw/misc: Fix arith overflow in NPCM7XX PWM module

hw/misc: Fix arith overflow in NPCM7XX PWM module

Fix potential overflow problem when calculating pwm_duty.
1. Ensure p->cmr and p->cnr to be from [0,65535], according to the
   hardware specification.
2. Changed duty to uint32_t. However, since MAX_DUTY * (p->cmr+1)
   can excceed UINT32_MAX, we convert them to uint64_t in computation
   and converted them back to uint32_t.
   (duty is guaranteed to be <= MAX_DUTY so it won't overflow.)

Fixes: CID 1442342
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Doug Evans <dje@google.com>
Signed-off-by: Hao Wu <wuhaotsh@google.com>
Message-id: 20210127011142.2122790-1-wuhaotsh@google.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>