Added alternative control mode.

It is now possible to control the motor by PWM-ing
the disable pins of the motor controller.
It is an ugly preprocessor hack, so:

Hic sunt dracones.

Signed-off-by: Anton Landberg <git@atnon.se>
Fixed duty cycle problem

Using DISABLE_PWM = 1 one must invert the duty cycle,
since the disable pin is active high.
This means that the motor is stopped when the duty cycle is 0xFF.

Signed-off-by: Anton Landberg <git@atnon.se>
Adjusted PWM.

/256 prescaler and inital behavour is DC=0xFF.

Signed-off-by: Anton Landberg <git@atnon.se>