brcmfmac: Fix iovar 'bw_cap' set command failure

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: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Daniel Kim <dekim@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>