ath9k: fix invalid descriptor discarding

ath9k: fix invalid descriptor discarding

Only set sc->rx.discard_next to rx_stats->rs_more when actually
discarding the current descriptor.

Also, fix a detection of broken descriptors:
First the code checks if the current descriptor is not done.
Then it checks if the next descriptor is done.
Add a check that afterwards checks the first descriptor again, because
it might have been completed in the mean time.

This fixes a regression introduced in
commit 849aedee730d53f180bf944e20b73d4c117b2a83
"ath9k: fix handling of broken descriptors"

Cc: stable@vger.kernel.org
Reported-by: Marco André Dinis <marcoandredinis@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>