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/sched.h

@@ -15,7 +15,7 @@
 // Timer structure for scheduling timed events (see sched_timer() )
 struct timer {
     struct timer *next;
-    uint8_t (*func)(struct timer*);
+    uint_fast8_t (*func)(struct timer*);
     uint32_t waketime;
 };