rpmsg: virtio_rpmsg_bus: fix announce for devices without endpoint

A device might not have an endpoint assigned. This patch checks if
rpdev->ept has a value before dereferencing or using it.

Signed-off-by: Henri Roosen <henri.roosen@ginzinger.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>