iwlwifi: use generic mac80211 radiotap headers

remove drivers own implementation of radiotap in favor of the generic one
provided by mac80211.

also remove priv->add_radiotap because it is not used any more.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>