brcmfmac: detect firmware support for monitor interface

brcmfmac: detect firmware support for monitor interface

Many/most of firmwares support creating monitor interface but only the
most recent ones explicitly /announce/ it using a "monitor" entry in the
list of capabilities.

Check for that entry and store internally info about monitor mode
support using a new feature flag. Once we sort out all details of
handling monitor interface it will be used when reporting available
interfaces to the cfg80211.

Later some fallback detecion method may be added for older firmwares.
For now just stick to the "monitor" capability which should be 100%
reliable.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>