pbio/motor_process: Reset timer instead of restart.

For the observer model, it is more important for the loop time to be
constant on average.

This reverts commit 2174461.

See pybricks/support#955

lib/pbio/src/motor_process.c

@@ -38,7 +38,7 @@ PROCESS_THREAD(pbio_motor_process, ev, data) {
         pbio_servo_update_all();
 
         // Reset timer to wait for next update
-        etimer_restart(&timer);
+        etimer_reset(&timer);
     }
 
     PROCESS_END();