Fixed PWM driver reset

The previous implementation only set the `MODE1` register to `0x00`, which is not the default of the _PCA9685_ (bit 0, `ALLCALL`, defaults to 1 - see section 7.3.1 in the datasheet) and ignores all other register like `MODE2`, all output values, etc. The _PCA9685_ offers a dedicated software reset, which _"allows all the devices in the I2C-bus to be reset to the power-up state"_ (section 7.6).