net: dsa: felix: Fix memory leak in felix_setup_mmio_filtering

Avoid a memory leak if there is not a CPU port defined.

Fixes: 8d5f7954

 ("net: dsa: felix: break at first CPU port during init and teardown")
Addresses-Coverity-ID: 1492897 ("Resource leak")
Addresses-Coverity-ID: 1492899 ("Resource leak")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211209110538.11585-1-jose.exposito89@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>