net: dsa: return early if there is no master

It doesn't make sense to have DSA without a master port. Error out early
if there is no master port.

Fixes: fc054d56 ("net: Introduce DSA class for Ethernet switches")
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>