Fixed I2C reading problems

[Okarin99]

This PR fixes some issues I had with I2C.
I wanted to query exactly 2 bytes over I2C, but I got an I2C arbitration error.
Because of this I fixed the code to follow exactly the rules described here.
Now it is working fine for me.

Example code:

#![no_std]
#![no_main]

use cortex_m::iprintln;
use cortex_m_rt::entry;
use mpu6050::*;
use stm32f1xx_hal::{delay::Delay, i2c, prelude::*, stm32};

#[allow(unused_imports)]
use panic_itm;

#[entry]
fn main() -> ! {
    let device = stm32::Peripherals::take().unwrap();
    let mut cp = cortex_m::Peripherals::take().unwrap();
    let stim = &mut cp.ITM.stim[0];

    let mut flash = device.FLASH.constrain();
    let mut rcc = device.RCC.constrain();

    let clocks = rcc.cfgr.freeze(&mut flash.acr);
    let mut afio = device.AFIO.constrain(&mut rcc.apb2);

    let mode = i2c::Mode::Standard {
        frequency: 100_000.hz(),
    };

    let mut gpiob = device.GPIOB.split(&mut rcc.apb2);

    let scl_pin = gpiob.pb8.into_alternate_open_drain(&mut gpiob.crh);
    let sda_pin = gpiob.pb9.into_alternate_open_drain(&mut gpiob.crh);

    let delay = Delay::new(cp.SYST, clocks);

    let i2c_val = i2c::BlockingI2c::i2c1(
        device.I2C1,
        (scl_pin, sda_pin),
        &mut afio.mapr,
        mode,
        clocks,
        &mut rcc.apb1,
        50000,
        10,
        50000,
        50000,
    );

    let mut mpu = Mpu6050::new(i2c_val, delay);
    mpu.init().unwrap();
    mpu.soft_calib(Steps(100)).unwrap();
    mpu.calc_variance(Steps(50)).unwrap();

    iprintln!(stim, "Calibrated with bias: {:?}", mpu.get_bias().unwrap());
    iprintln!(
        stim,
        "Calculated variance: {:?}",
        mpu.get_variance().unwrap()
    );

    loop {
        /*
        // Didn't work before
        let mut buf: [u8; 2] = [0; 2];
        mpu.read_bytes(0x41, &mut buf).unwrap();
        iprintln!(stim, "{:?}", buf);
        */

        /*
        let acc = mpu.get_acc().unwrap();
        let gyro = mpu.get_gyro().unwrap();
        let temp = mpu.get_temp().unwrap();
        iprintln!(stim, "Acc: {:?}; Gyro: {:?}; Temp: {:?}", acc, gyro, temp);
        */

        let acc = mpu.get_acc_angles_avg(Steps(200)).unwrap();
        iprintln!(stim, "Acc: {:?}", acc);
    }
}

[TheZoq2]

Thanks for the PR, and sorry for the review delay :)

The code looks great!

A comment pointing to the document you based it on in the code and a changelog entry can't hurt, I can take care of that :)