ath6kl: Add debugfs files for roaming control

Roaming mode can be changed by writing roam mode (default, bssbias, or
lock) to roam_mode. Forced roam can be requested by writing the BSSID
into force_roam.

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