ath6kl: pass only unicast frames for aggregation

When pinging form ar6003 to the AP RTT was high even when power save was
disabled:

100 packets transmitted, 97 received, 3% packet loss, time 99125ms
rtt min/avg/max/mdev = 1.875/46.733/795.506/139.181 ms

After some investigation one reason for this was that received
multicast traffic confused the aggrecation logic and caused 400 ms
timeouts when receiving multicast frames from AP.

A simple way to fix is to pass only unicast frames for aggregation. This
improves RTT:

100 packets transmitted, 99 received, 1% packet loss, time 99144ms
rtt min/avg/max/mdev = 2.083/13.084/403.390/56.794 ms

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>