ath6kl: add firmware log support

Firmware sends binary logs with WMIX_DBGLOG_EVENTID event. Create
a buffer which stores the latest logs and which can be copied from
fwlog debugfs file with cp command.

To save memory firmware log support is enabled only when CONFIG_ATH6KL_DEBUG
is enabled.

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