Fix iovar 'bw_cap' set command failure introduced by

  commit ff3b0fba6f25555ef59c55d138a467d0f81d82d7
  Author: Arend van Spriel <arend@broadcom.com>
  Date:   Sat Mar 15 12:00:57 2014 +0100

      brcmfmac: fallback to mimo_bw_cap for older firmwares

This resulted in disabling 20MHz operation in the firmware.
Reviewed-by: NArend Van Spriel <arend@broadcom.com>
Signed-off-by: NDaniel Kim <dekim@broadcom.com>
Signed-off-by: NArend van Spriel <arend@broadcom.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>