net: dsa: remove NULL check for priv and platform data

Because the uclass has the "*_auto" properties set, the driver model
will take care of allocating the private structures for us and they
can't be NULL. Drop the checks.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>