cfg80211: fix parsing when db.txt ends on a rule

If genregdb.awk assumes the file will end with an
extra empty line or a comment line. This is could
not be true so just address this.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Acked-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>