ath9k: use a separate data structure for rx buffers

There's no shared code for handling both rx and tx buffers, and tx
buffers require a lot more metadata than rx buffers.
Using a separate data structure for rx reduces memory usage and improves
cache footprint.

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