Similar to Felix Fietkau <nbd@openwrt.org> "ath9k_hw: optimize all descriptor
access functions" (13db2a80244908833502189a24de82a856668b8a).
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>