mac80211: remove RX_FLAG_RADIOTAP

While there may be a case for a driver adding its
own bits of radiotap information, none currently
does. Also, drivers would have to copy the code
to generate the radiotap bits that now mac80211
generates. If some driver in the future needs to
add some driver-specific information I'd expect
that to be in a radiotap vendor namespace and we
can add a different way of passing such data up
and having mac80211 include it.

Additionally, rename IEEE80211_CONF_RADIOTAP to
IEEE80211_CONF_MONITOR since it's still used by
b43(legacy) to obtain per-frame timestamps.

The purpose of this patch is to simplify the RX
code in mac80211 to make it easier to add paged
skb support.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>