brcmfmac: Limit control message length from host to device.

An control request or set message length is restricted to
ETH frame length for the buffer from host to device. This
is limitation is imposed by the firmware.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>