ath9k: Fix sequence number assignment for non-data frames

Since commit 558ff225 (ath9k: fix
ps-poll responses under a-mpdu sessions) non-data frames would have
gotten a sequence number from a TIDs sequence counter instead of
using the global sequence counter.

This can lead to instable connections.

To fix this only select the correct TID if we are processing a
data frame. Furthermore, prevent non-data frames to get a sequence
number from a TID sequence counter by adding a check to
ath_tx_setup_buffer.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>