ath9k: fix regression in sending aggregated packets

The recent commit "ath9k: Send legacy rated frames as unaggregated"
introduced a check to ensure that packets with non-MCS rates set in
the rate series will not be aggregated. However, it failed to check
if the rate series is valid before testing the flags, thus breaking
aggregation for normal MCS-only packets if the last series is unset.

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