ath10k: start using sk_buff_head

Instead of using manual sk_buff linking via ->next
use sk_buff_head. It's more robust, cleaner and
there's plenty of helper functions in kernel
already to manage sk_buff_head.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>