net: dsa: if ds->setup is true, ds->devlink is always non-NULL

Simplify dsa_switch_teardown() to remove the NULL checking for
ds->devlink.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>