mac80211: make rx radiotap header more flexible

use hw flags and rx flags to determine which fields are present in the header
and use all available information from the driver.

make sure radiotap header starts at a naturally aligned address (mod 8) for
all radiotap fields.

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