rpmsg: Only invoke announce_create for rpdev with endpoints

For special rpmsg devices without a primary endpoint there is nothing to
announce so don't call the backend announce create function if we didn't
create an endpoint.

Change-Id: I42adaa317e408b73061f1c3d392699bec36b193e
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-msm @ 27/03/2018, 23:07
Signed-off-by: Chris Lew <clew@codeaurora.org>