[net80211] add support for drivers to disable sending NULL data frames

[adrian]

Summary

net80211 has various places where null data / null qos data frames
are sent. However plenty of NICs shouldn't be sending them from
net80211 and it may even upset their 802.11n window / sequence number
tracking.

So add support here.

[bz]

sys/net80211/ieee80211_output.c

1085

Comments end in a .

1086

style.9: if ((ic->ic_flags_ext & IEEE80211_FEXT_NO_NULLDATA) != 0)