[net80211] add a "is VHT available" macro.

We have run out of config bits, sigh, so until I expand the ic config
bits, just use this macro as a substitute.

sys/net80211/ieee80211_var.h

@@ -87,6 +87,14 @@
 /* XXX TODO: cap this at 1, in case hz is not 1000 */
 #define	IEEE80211_TU_TO_TICKS(x)(((uint64_t)(x) * 1024 * hz) / (1000 * 1000))
 
+/*
+ * Technically, vhtflags may be 0 /and/ 11ac is enabled.
+ * At some point ic should just grow a flag somewhere that
+ * says that VHT is supported - and then this macro can be
+ * changed.
+ */
+#define	IEEE80211_CONF_VHT(ic)		((ic)->ic_vhtcaps != 0)
+
 /*
  * 802.11 control state is split into a common portion that maps
  * 1-1 to a physical device and one or more "Virtual AP's" (VAP)