remoteproc: Fix an error code in devm_rproc_alloc()

The comments say that this function should return NULL on error and the
caller expects NULL returns as well so I have modified the code to match.
Returning an ERR_PTR(-ENOMEM) would lead to an OOps.
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Fixes: 305ac5a7 ("remoteproc: Add device-managed variants of rproc_alloc/rproc_add")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200520120705.GH172354@mwandaSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -2297,7 +2297,7 @@ struct rproc *devm_rproc_alloc(struct device *dev, const char *name,
 
 	ptr = devres_alloc(devm_rproc_free, sizeof(*ptr), GFP_KERNEL);
 	if (!ptr)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
 	rproc = rproc_alloc(dev, name, ops, firmware, len);
 	if (rproc) {