wireless: fixup genregdb.awk for remove of antenna gain from wireless-regd

Since "wireless-regdb: remove antenna gain" was merged in the
wireless-regdb tree, the awk script parser has been incompatible
with the 'official' regulatory database.  This fixes that up.
Without this change the max EIRP is set to 0 making 802.11 devices
useless.

The fragile nature of the awk parser must be replaced, but ideas
over how to do that in the most scalable way are being reviewed.
In the meantime update the documentation for CFG80211_INTERNAL_REGDB
so folks are aware of expectations for now.

Reported-by: John Walker <john@x109.net>
Reported-by: Krishna Chaitanya <chaitanya.mgit@gmail.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>