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>