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>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>