ath9k: use the new API for setting tx descriptors

With the new API, tx descriptors can be written in one single pass
instead of having to re-read and rewrite fields from multiple places.
This makes the code easier to read and also slightly improves performance
on embedded MIPS hardware.

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