drivers: pwm: stm32: fix four channel pwm capture

With the four-channel-capture-support property enabled, the STM32 PWM
driver was missing every other frames during PWM capture.

The counter values are now reset just after getting the pulse and the
period of the cycle, instead of waiting the next interrupt to do so, and
thus missing a cycle.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>