ath9k: implement buffer holding handling for EDMA FIFO

Inside one FIFO slot queue, EDMA chipsets have the same link pointer
re-read race condition as older chipsets, so the same buffer holding
logic needs to be used in order to avoid use-after-free bugs.
Unlike on older chips, it can be skipped for the end of the queue.

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