Fix MonoTimer and Timer::syst frequency

[TheZoq2]

Continuation of #191, broken into its own PR because I didn't want to force push to someone elses master branch 😉

After some correct testing, I reproduced the bug. I also noticed the same problem with Timer::syst using the following code:

#![no_main]
#![no_std]

use panic_halt as _;

use stm32f1xx_hal as hal;

use crate::hal::{
    gpio::{gpioc, Output, PushPull},
    pac::{Peripherals, CorePeripherals, TIM2},
    prelude::*,
    timer::{CountDownTimer, Event, Timer},
    time::MonoTimer,
};

use core::cell::RefCell;
use cortex_m::{asm::wfi, interrupt::Mutex};
use cortex_m_rt::entry;
use embedded_hal::digital::v2::OutputPin;
use nb::block;
use cortex_m_semihosting::hprintln;

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

    let mut rcc = dp.RCC.constrain();
    let mut flash = dp.FLASH.constrain();
    let clocks = rcc
        .cfgr
        .sysclk(16.mhz())
        .hclk(4.mhz())
        .freeze(&mut flash.acr);

    hprintln!("sysclk {}", clocks.sysclk().0);
    hprintln!("hclk {}", clocks.hclk().0);

    let mono = MonoTimer::new(cp.DWT, clocks);

    let mut timer = Timer::syst(cp.SYST, &clocks)
        .start_count_down(1.hz());

    let mut start = mono.now();

    loop {
        block!(timer.wait()).unwrap();
        hprintln!("{}", start.elapsed() as f32 / mono.frequency().0 as f32).unwrap();
        timer.start(1.hz());
        start = mono.now();
    }
}

which uses the now functioning MonoTimer to measure the incorrectness of syst, which is now 4 times slower than intended.

cc: @ra-kete

[therealprof]

LGTM