In each iteration fwnode_for_each_available_child_node() bumps a reference
counting of a loop variable followed by dropping in on a next iteration,

Since in error case the loop is broken, we have to drop a reference count
by ourselves. Do it for port_fwnode in error case during ->probe().

Fixes: 24812221 ("net: mvpp2: use device_*/fwnode_* APIs instead of of_*")
Cc: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>