sched: Use uint_fast8_t for return type of timers

Some architectures are faster passing regular integers than 8bit
integers.  Use uint_fast8_t so that the architecture chooses the
appropriate type.

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

src/stepper.h

@@ -5,7 +5,7 @@
 
 enum { STEPPER_POSITION_BIAS=0x40000000 };
 
-uint8_t stepper_event(struct timer *t);
+uint_fast8_t stepper_event(struct timer *t);
 void command_config_stepper(uint32_t *args);
 struct stepper;
 uint32_t stepper_get_position(struct stepper *s);