mac80211: move fragment flag to info flag as dont-fragment

The purpose of this is two-fold:
 1) by moving it out of tx_data.flags, we can in
    another patch move the radiotap parsing so it
    no longer is in the hotpath
 2) if a device implements fragmentation but can
    optionally skip it, the radiotap request for
    not doing fragmentation may be honoured

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