ath6kl: Fix buffer alignment for scatter-gather I/O

For non-scatter buffers, there is already a bounce buffer which
takes care of alignment. This patch is influenced by a rough patch of
Kalle.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>