drivers: stepper: Change stepper velocity to step interval

Change the stepper API to instead of changing the stepper speed based on
the velocity in microsteps per second to use the delay in usec between
successive steps. Also remove the velocity from the `stepper_run` function
as typical API usage is enable -> set step interval -> run.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>