greybus: es1-ap-usb: adjust SVC buffer size to handle worst case

The worst case message from the SVC->AP is a hotplug "plugged"
event. It includes the module manifest which may be up to 64KB
in size. Adjust our buffer allocation to allow for this.

Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>