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

include/linux/rpmsg.h

@@ -231,7 +231,7 @@ static inline struct rpmsg_endpoint *rpmsg_create_ept(struct rpmsg_device *rpdev
 	/* This shouldn't be possible */
 	WARN_ON(1);
 
-	return ERR_PTR(-ENXIO);
+	return NULL;
 }
 
 static inline int rpmsg_send(struct rpmsg_endpoint *ept, void *data, int len)