ksmbd: use netif_is_bridge_port

Use netif_is_bridge_port defined in <linux/netdevice.h> instead of
open-coding it.

Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Steve French <stfrench@microsoft.com>