[pwm, rtl] PWM RTL fix for issue mentioned in PR#13295
[pwm, rtl] PWM RTL fix for issue mentioned in PR#13295

  - Fix the duty cycle state transistion issue in heart beat mode

Signed-off-by: Muqing Liu <b1c0f8d5c8fd5df55dfdf439926eb15b5e395e02@wdc.com>
Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@opentitan.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan