ath6kl: Use cfg80211_roamed_bss() to report roaming event

This is to avoid the scenario where the bss entry of the AP got
expired when reporting roaming event to current AP. As the bss
entry for the current bss is available in driver, pass this bss
to cfg80211. This fixes WARNING: at net/wireless/sme.c:586.

This patch depends on the following patch in cfg80211
"cfg80211: Fix race in bss timeout".

Reported-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>