ath6kl: Collect residue firmware logs

Collect residue firmware logs following firmware assert.
Firmware sends logs to host once the 1500 byte log buffer
has been filled. At time of assert, there could be residue
logs lying in the firmware. This patch pulls those residue
logs. This would give the full picture of the firmware.

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