staging: fsl-dpaa2: ethsw: Introduce ethsw_port_dev_check()

ethsw currently uses an open-coded comparison of netdev_ops to determine
whether whether a device represents a front panel port. Wrap this into a
named function to simplify reuse.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>