net: xen-netback: xenbus: Demote nonconformant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/net/xen-netback/xenbus.c:419: warning: Function parameter or member 'dev' not described in 'frontend_changed'
 drivers/net/xen-netback/xenbus.c:419: warning: Function parameter or member 'frontend_state' not described in 'frontend_changed'
 drivers/net/xen-netback/xenbus.c:1001: warning: Function parameter or member 'dev' not described in 'netback_probe'
 drivers/net/xen-netback/xenbus.c:1001: warning: Function parameter or member 'id' not described in 'netback_probe'
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/xen-netback/xenbus.c

@@ -411,7 +411,7 @@ static void read_xenbus_frontend_xdp(struct backend_info *be,
 	vif->xdp_headroom = headroom;
 }
 
-/**
+/*
  * Callback received when the frontend's state changes.
  */
 static void frontend_changed(struct xenbus_device *dev,
@@ -996,7 +996,7 @@ static int netback_remove(struct xenbus_device *dev)
 	return 0;
 }
 
-/**
+/*
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures and switch to InitWait.
  */