ath6kl: Indicate the roaming capability of the firmware

When the rssi of the current AP drops, both wpa_supplicant and the
firmware may do a background scan to find a better AP and try to
associate. This might lead to a race condition where both may try
to connect to some AP based on their scan results.

Since the firmware is capable of handling roaming, let
wpa_supplicant know about this capability so that it will back off
from bgscan based roaming.

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