Explicitly mark constant as 'unsigned' to avoid unnecessary promotion…

… to 'long'.

Long constants trigger the performance warning "Constant is long" with cc65.

core/net/mac/tsch/tsch-conf.h

@@ -135,7 +135,7 @@
 #define TSCH_DEFAULT_TS_MAX_TX             4256
 #define TSCH_DEFAULT_TS_TIMESLOT_LENGTH    15000
 
-#elif TSCH_CONF_DEFAULT_TIMESLOT_LENGTH == 65000
+#elif TSCH_CONF_DEFAULT_TIMESLOT_LENGTH == 65000U
 /* 65ms timeslot, i.e. nearly the max length allowed by standard (16-bit unsigned in micro-seconds).
  * Useful for running link-layer security on sky or z1 in Cooja, where only S/W security is supported.
  * Note: this slot timing would require a total of 120ms. If a slot overlaps with the next active slot,