greybus: core: fix hd-creation error path

Make sure to return an errno when a host-device buffer-size check fails.

Fixes: 1f92f6404614 ("core: return error code when creating host device")
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/staging/greybus/core.c

@@ -184,7 +184,7 @@ struct greybus_host_device *greybus_create_hd(struct greybus_host_driver *driver
 
 	if (buffer_size_max < GB_OPERATION_MESSAGE_SIZE_MIN) {
 		dev_err(parent, "greybus host-device buffers too small\n");
-		return NULL;
+		return ERR_PTR(-EINVAL);
 	}
 
 	if (num_cports == 0 || num_cports > CPORT_ID_MAX) {