Revert "PCI: ecam: Allow cfg->priv to be pre-populated from the root port device"

This reverts commit 4900454b4f819e88e9c57ed93542bf9325d7e161.

Now that nobody relies of cfg->priv containing anything useful before
the .init() callback is used, restore the previous behaviour.

Signed-off-by: Marc Zyngier <maz@kernel.org>