brcmfmac: Add RSDB support.

Broadcom devices with a single 802.11 core can work on two band
concurrently using VSDB feature, ie. Virtual Simultaneous Dual-Band.
For devices that are fitted with two 802.11 cores and RF paths the
driver should support a firmware feature called RSDB, which stands
for Real Simultaneous Dual-Band. RSDB works almost autonomously in
firmware except for AP config. When the device supports RSDB then
the interface should not be brought down when configuring it,
otherwise the link (if configured) on the other interface will be
lost.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
[kvalo@codeaurora.org: changed the commit log based on discussion]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>