use the rates ERP flag to derive CCK or OFDM modulation for the radiotap
header.

(it might be more correct to get this information from the hardware itself, but it
seems safe to assume this in most practical cases.)
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>