net: switchdev: Fixed kerneldoc warning

Update kernel-doc line comments to fix warnings reported by make W=1.
net/switchdev/switchdev.c:413: warning: Function parameter or
member 'extack' not described in 'call_switchdev_notifiers'

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>