rpmsg: Fix rpmsg_create_ept return when RPMSG config is not defined

According to the description of the rpmsg_create_ept in rpmsg_core.c
the function should return NULL on error.

Fixes: 2c8a5708 ("rpmsg: Provide function stubs for API")
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210712123912.10672-1-arnaud.pouliquen@foss.st.com


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>