Lower chopper frequency for TMC2240 may not be required

[liampwll]

This recent commit changed the target PWM frequency of the TMC2240 to 20kHz to reduce the power dissipation: fd68808

I suspect that any increased power dissipation compared to the TMC2209 is due to the output slew control that defaults to 100V/us but can be increased up to 800V/us, giving us a faster slew rate than the 2209.

If we assume a motor running at 2A and 24V the dissipation can be reduced by an entire watt at 55kHz, likely allowing the chopper frequency to be increased to match the other drivers:

55kHz×2A×(24V/2)×(24V/(100V/us))×4 = 1.2672W
55kHz×2A×(24V/2)×(24V/(200V/us))×4 = 0.6336W
55kHz×2A×(24V/2)×(24V/(400V/us))×4 = 0.3168W
55kHz×2A×(24V/2)×(24V/(800V/us))×4 = 0.1584W