Documentation: networking: switchdev: Update port parent ID section

Update the section about switchdev drivers having to implement a
switchdev_port_attr_get() function to return
SWITCHDEV_ATTR_ID_PORT_PARENT_ID since that is no longer valid after
commit bccb3025 ("net: Get rid of
SWITCHDEV_ATTR_ID_PORT_PARENT_ID").

Fixes: bccb3025 ("net: Get rid of SWITCHDEV_ATTR_ID_PORT_PARENT_ID")
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/switchdev.txt

@@ -92,11 +92,11 @@ device.
 Switch ID
 ^^^^^^^^^
 
-The switchdev driver must implement the switchdev op switchdev_port_attr_get
-for SWITCHDEV_ATTR_ID_PORT_PARENT_ID for each port netdev, returning the same
-physical ID for each port of a switch.  The ID must be unique between switches
-on the same system.  The ID does not need to be unique between switches on
-different systems.
+The switchdev driver must implement the net_device operation
+ndo_get_port_parent_id for each port netdev, returning the same physical ID for
+each port of a switch. The ID must be unique between switches on the same
+system. The ID does not need to be unique between switches on different
+systems.
 
 The switch ID is used to locate ports on a switch and to know if aggregated
 ports belong to the same switch.