wcn36xx: move set_tx_pdu inside set_tx_data/mgmt

The pdu is part of the buffer descriptor, so it makes
sense that one function would fill both.  Also, passing
the whole skb instead of just the header pointer to the
set_tx_data function paves the way for using its fields
for ampdu setup inside set_tx_data().

Signed-off-by: Bob Copeland <me@bobcopeland.com>
wcn36xx: initiate TX BA sessions

Currently, wcn36xx only asks for a TX BA session if it has
already established one for RX.  Thus, two wcn36xx devices cannot
do a-mpdu between themselves since they both wait for the other
to go first.  Fix this by starting a BA session after a few QoS
data frames have been sent to a STA.

Signed-off-by: Bob Copeland <bob@cozybit.com>