ath9k: Last fix for TX software padding.

First, we copy/paste the padding stuff from ath9k_tx to ath_tx_cabq since it
needs to same kind of padding, but for internally generated beacons.
Next, software padding done on TX needs to be removed before calling
ieee80211_tx_status. The code was already there in ath_tx_complete but it
was wrong. Fix it by using ath9k_cmn_padpos. This later code has been
tested by sending packets to a monitor interface and reading packets from the
same interface.

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
Signed-off-by: John W. Linville <linville@tuxdriver.com>