brcmfmac: fallback to mimo_bw_cap for older firmwares

In order to support the driver behaviour introduced by:

   commit d0575a5a703978c43e25128421158c78534ba100
   Author: Daniel Kim <dekim@broadcom.com>
   Date:   Wed Mar 12 18:12:14 2014 -0700

       brcmfmac: Enable 40MHz bandwidth in 2GHz band and OBSS scanning

in devices that do not support bwcap firmware command a fallback
is added.

Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>