ath6kl: change aggreation timeout message from an error to a debug message

When I connect to my Linksys WT610N AP supporting 11n I see a lot of
aggreation timeout errors:

[  408.885053] ath6kl: aggr timeout (st 3109 end 3140)
[  463.872108] ath6kl: aggr timeout (st 3671 end 3702)
[  495.010060] ath6kl: aggr timeout (st 3983 end 4014)
[  503.604047] ath6kl: aggr timeout (st 4065 end 0)
[  518.963047] ath6kl: aggr timeout (st 141 end 172)
[  525.014066] ath6kl: aggr timeout (st 205 end 236)
[  573.957051] ath6kl: aggr timeout (st 701 end 732)
[  585.019067] ath6kl: aggr timeout (st 816 end 847)

But still the connection seems to work. To not clutter the logs change
the error message to a debug message. But add a fixme comment so that
this will be investigated.

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