brcmfmac: add initial support for monitor mode

Report monitor interface availability using cfg80211 and support it in
the add_virtual_intf() and del_virtual_intf() callbacks. This new
feature is conditional and depends on firmware flagging monitor packets.
Receiving monitor frames is already handled by the brcmf_netif_mon_rx().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>