I2C & PWM fixes

[jesstr]

1. Fixed linux i2cdetect tool can't detect the board;
2. Fixed PWM frequency set according to the issue PWM frequency and steps are set incorectly #9. Checked with the oscilloscope - works like a charm, thanks @danielfmo; Max PWM frequency is limited to the 31250Hz.
3. Changed PWM duty cycle set format to percents (0-100%). This fix can break some workflows with the old code, but the frequency/duty cycle set commands was actually broken anyway, so I think more intuitive motor speed control worth its incompatibility.