ath9k: do not insert padding into tx buffers on AR9380+

With the new EDMA descriptor format, a single descriptor can contain up
to four buffer pointers. By splitting the buffer into two parts, we can
let the hardware add the padding internally instead of using memmove on
the skb data.

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