ath9k_hw: add a new API for setting tx descriptors

Instead of using lots of different functions with long argument lists,
pull all the necessary information from one struct. This makes the code
easier to read and eliminates the need for copying data between multiple
linked descriptors.

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