wl12xx: Remove private headers in wl1271_tx_reset

Frames in the tx_frames array include extra private headers, which must
be removed before passing the skbs to ieee80211_tx_status.

Fix this by removing any private headers in wl1271_tx_reset, similar to
how this is done in wl1271_tx_complete_packet.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>