New Broadcom firmwares mark monitor mode packets using a newly defined
bit in the flags field. Use it to filter them out and pass to the
monitor interface. These defines were found in bcmmsgbuf.h from SDK.

As not every firmware generates radiotap header this commit introduces
BRCMF_FEAT_MONITOR_FMT_RADIOTAP flag. It has to be has based on firmware
capabilities. If not present brcmf_netif_mon_rx() will assume packet is
a raw 802.11 frame and will prepend it with an empty radiotap header.

This new code is limited to the msgbuf protocol at this point. Adding
support for SDIO/USB devices will require some extra work (possibly a
new firmware release).
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>