ath9k: fix more-data flag for buffered multicast packets

The flag needs to be cleared for the last packet in the list, not the
first one. Fixes some issues with multicast packet loss for powersave
clients connected to an ath9k AP.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>