ath6kl: unblock fwlog_block_read() on exit

Complete the waiting fwlog_block_read on debugfs cleanup, otherwise
userspace and module unload might softlock.

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