ath9k: fix setting the IEEE80211_TX_CTL_CLEAR_PS_FILT flag

When the driver inserts padding between the 802.11 header and data, it
needs to set the hdr variable to the new header location.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>