net: Get rid of switchdev_port_attr_get()

With the bridge no longer calling switchdev_port_attr_get() to obtain
the supported bridge port flags from a driver but instead trying to set
the bridge port flags directly and relying on driver to reject
unsupported configurations, we can effectively get rid of
switchdev_port_attr_get() entirely since this was the only place where
it was called.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>