sched: Change sched_from_ms() to sched_from_us()

Some code may require micro-second precision so update sched_from_ms()
to use micro-seconds.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/simulator/main.c

@@ -17,7 +17,7 @@ uint8_t Interrupt_off;
  ****************************************************************/
 
 uint32_t
-timer_from_ms(uint32_t ms)
+timer_from_us(uint32_t us)
 {
     return 0; // XXX
 }