mac80211: do not convert to A-MSDU if frag/subframe limited

Do not start to aggregate packets in a A-MSDU frame (converting the
first subframe to A-MSDU, adding the header) if max_tx_fragments or
max_amsdu_subframes limits are already exceeded by it. In particular,
this happens when drivers set the limit to 1 to avoid A-MSDUs at all.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
[reword commit message to be more precise]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>