rpmsg: process _all_ pending messages in rpmsg_recv_done

Change virtqueue callback function rpmsg_recv_done() to process all
available messages instead of just one message.

Signed-off-by: Robert Tivy <rtivy@ti.com>
[split _recv function instead of adding indentation]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>