the logical thing to do would be:
- set enable pin high
- set one control pin low/high depending on direction
- pulse other control pin with analogWrite(controlpin, value)
for speed control.
You might also just be able to leave the control signal as is, and pulse the enable pin with the analogWrite PWM signal.
Watch out for heat. With all that swtiching going on, the motor will throw a lot of back emf into your driver chip and the protection diodes.
Don't be surprised if it gets really hot or self-destructs as a result. The solution to this is good bypass capacitors and sometimes even low-value inductors in series with the motor. A big wheelchair-type motor will take some careful design for the switching noise. Small hobby-type motors will be no sweat.