wil6210: multiple VIFs support for start/stop AP

Add support for multiple VIFs in the cfg80211 operations start_ap,
stop_ap and change_beacon. This change allows starting multiple APs
using virtual interfaces.
The data path and most other operations are still working only
on the main interface.

Signed-off-by: Lior David <liord@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>