Reset timer CNT register when PWM period is changed

[scd31]

When changing the period of a PWM timer repeatedly, it will sporadically stop working. This is because the CNT register isn't being reset.

Minimum reproducible example:

#![no_main]
#![no_std]

// use panic_halt as _;
use panic_semihosting as _;

use cortex_m_rt::entry;
use stm32f4xx_hal as hal;

use crate::hal::{pac, prelude::*, timer::Channel};

#[entry]
fn main() -> ! {
    let dp = pac::Peripherals::take().unwrap();

    let buzzer = dp.GPIOA.split().pa0.into_alternate();

    // Set up the system clock. We want to run at 48MHz for this one.
    let rcc = dp.RCC.constrain();
    let clocks = rcc.cfgr.use_hse(25.MHz()).sysclk(48.MHz()).freeze();

    let mut pwm = dp.TIM5.pwm_hz(buzzer, 1000.Hz(), &clocks);
    pwm.set_duty(Channel::C1, pwm.get_max_duty() / 2);
    pwm.enable(Channel::C1);

    let mut delay = dp.TIM1.delay_us(&clocks);

    loop {
        pwm.set_period(1000.Hz());
        pwm.set_duty(Channel::C1, pwm.get_max_duty() / 2);
        pwm.configure(&clocks);

        delay.delay_ms(1000_u32);

        pwm.set_period(2000.Hz());
        pwm.set_duty(Channel::C1, pwm.get_max_duty() / 2);
        pwm.configure(&clocks);

        delay.delay(1.secs());
    }
}

Before this PR, this would work for a few cycles before the signal disappears. It would occasionally reappear every few minutes. After this PR, the code is consistently outputting a signal that alternates between 1 and 2 KHz.

[burrbull]

Should we also clear interrupt flags there?

Add this to Pwm::set_period also and update changelog

[scd31]

@burrbull I am not sure if we want to clear the interrupt flags. I would imagine we'd want to keep them (if an interrupt is raised right before set_period is called we probably still want to handle that interrupt)

[burrbull]

bors r+

[bors]

Build succeeded:

• ci (stm32f401, stable)
• ci (stm32f405, stable)
• ci (stm32f407, stable)
• ci (stm32f410, stable)
• ci (stm32f411, stable)
• ci (stm32f412, stable)
• ci (stm32f413, stable)
• ci (stm32f415, stable)
• ci (stm32f417, stable)
• ci (stm32f423, stable)
• ci (stm32f427, stable)
• ci (stm32f429, stable)
• ci (stm32f437, stable)
• ci (stm32f439, stable)
• ci (stm32f446, stable)
• ci (stm32f469, stable)
• ci (stm32f479, stable)
• Rustfmt