ath9k: fix rate control fallback rate selection

When selecting the tx fallback rate, rc.c used a separate variable
'nrix' for storing the next rate index, however it did not use that as
reference for further rate index lowering. Because of that, it ended up
reusing the same rate for multiple multi-rate retry stages, thus
decreasing delivery probability under changing link conditions.

This patch removes the separate (unnecessary) variable and fixes
fallback the way it was intended to work.
This should result in increased throughput and better link stability.

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