mac80211: skip radiotap space calculation if no monitor exists

The radiotap header length "needed_headroom" is only required if we're
sending the skb to a monitor interface. Hence, move the calculation a
bit later so the calculation can be skipped if no monitor interface is
present.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>