[ath_hal] note that the CCA configuration setting may be chip-dependent.

I bet it isn't, but who knows - this is making assumptions about the
layout of AR_DIAG.

sys/dev/ath/ath_hal/ah.c

@@ -1415,6 +1415,9 @@ ath_hal_setcca(struct ath_hal *ah, int ena)
 
 /*
  * Get CCA setting.
+ *
+ * XXX TODO: turn this and the above function into methods
+ * in case there are chipset differences in handling CCA.
  */
 int
 ath_hal_getcca(struct ath_hal *ah)