swdevice: add new apis to set and del bridge port attributes

This patch adds two new api's netdev_switch_port_bridge_setlink
and netdev_switch_port_bridge_dellink to offload bridge port attributes
to switch port

(The names of the apis look odd with 'switch_port_bridge',
but am more inclined to change the prefix of the api to something else.
Will take any suggestions).

The api's look at the NETIF_F_HW_SWITCH_OFFLOAD feature flag to
pass bridge port attributes to the port device.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>