mac80211: don't call mgd_prepare_tx when associated

This doesn't make any sense since we are expected to be on
the medium or at least to Tx only when we are on the right
channel and the AP/GO can hear us.

Move the call to mgd_prepare_tx() for deauth to be only
done in case we're sending a deauth while not associated.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>