Fix kernel driver probe not freeing endpoint

If an error occurs after the kernel driver probe has created a rpmsg
endpoint, the endpoint is not freed during the clean up.

Clean up code for the endpoint has now been added to resolve this.

Change-Id: I9259713f2b7dcf4926e6f317c57f2791ab3463de
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>