ath6kl: store firmware logs in skbuffs

Currently firmware logs are stored in a circular buffer, but this was
not very flexible and fragile. It's a lot easier to store logs to struct
skbuffs and store them in a skb queue. Also this makes it possible
to easily increase the buffer size, even dynamically if we so want (but
that's not yet supported).

From user space point of view nothing should change.

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