mac80211: fixup radiotap tx flags for RTS/CTS

When using RTS/CTS, the CTS-to-Self bit in radiotap TX flags is
getting set instead of the RTS bit.  Set the correct one.

Reported-by: Larry Maxwell <larrymaxwell@agilemesh.com>
Signed-off-by: Bob Copeland <bob@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>