greybus: es1: fix buffer-size limit

The maximum buffer size does not include the headroom, so subtract the
headroom size from the actual buffer size.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>