ath6kl: add blocking debugfs file for retrieving firmware logs

When debugging firmware issues it's not always enough to get
the latest firmware logs, sometimes we need to get logs from a longer
period. To make this possible, add a debugfs file named fwlog_block. When
reading from this file ath6kl will send firmware logs whenever available
and otherwise it will block and wait for new logs.

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