1. The fault signal 3 for unit 2 is corrected to the right value
2. Now `mcpwm_fault_set_oneshot_mode` will clear the status before, no
need to reset the peripheral.
3. The capture feature relies on the prescaler, but it's only
initialized in the mcpwm_init funciton, which is used to initialize a
PWM channel. This means, the capture may not work correctly if no PWM
channel is enabled.
Now the prescaler is also updated when `mcpwm_capture_enable` is
called.
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