ath9k_htc: Restore skb headroom when returning skb to mac80211

commit d2e9fc141e2aa21f4b35ee27072d84e9aa6e2ba0 upstream.

ath9k_htc adds padding between the 802.11 header and the payload during
TX by moving the header. When handing the frame back to mac80211 for TX
status handling the header is not moved back into its original position.
This can result in a too small skb headroom when entering ath9k_htc
again (due to a soft retransmission for example) causing an
skb_under_panic oops.

Fix this by moving the 802.11 header back into its original position
before returning the frame to mac80211 as other drivers like rt2x00
or ath5k do.

Reported-by: Marc Kleine-Budde <mkl@blackshift.org>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Tested-by: Marc Kleine-Budde <mkl@blackshift.org>
Signed-off-by: Marc Kleine-Budde <mkl@blackshift.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>