nvmem: core: Properly handle connection ID in of_nvmem_device_get()

of_nvmem_device_get() would crash if NULL was passed as a connection
ID. Rework this to use the usual sementic of assuming the first
connection when no connection ID is given.

Furthermore of_nvmem_device_get() would return -EINVAL when it failed
to resolve the connection, making it impossible to properly implement
an optional connection. Return -ENOENT instead to let the caller know
that the connection doesn't exists.
Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/core.c

@@ -839,13 +839,14 @@ struct nvmem_device *of_nvmem_device_get(struct device_node *np, const char *id)
 {
 
 	struct device_node *nvmem_np;
-	int index;
+	int index = 0;
 
-	index = of_property_match_string(np, "nvmem-names", id);
+	if (id)
+		index = of_property_match_string(np, "nvmem-names", id);
 
 	nvmem_np = of_parse_phandle(np, "nvmem", index);
 	if (!nvmem_np)
-		return ERR_PTR(-EINVAL);
+		return ERR_PTR(-ENOENT);
 
 	return __nvmem_device_get(nvmem_np, NULL);
 }