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: 2c8a57088045 ("rpmsg: Provide function stubs for API")
Signed-off-by: Arnaud Pouliquen <[e-mail]>
Reviewed-by: Mathieu Poirier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bjorn Andersson <[e-mail]>

include/linux/rpmsg.h

···
 	/* 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)