More logic will be added to dsa_tree_setup_master() and
dsa_tree_teardown_master() in upcoming changes.

Reduce the indentation by one level in these functions by introducing
and using a dedicated iterator for CPU ports of a tree.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>