PCI: mvebu: increment nports only for registered ports

The number of ports is probed by counting the number of available child nodes.
Later on, the registration of a port can fail and cause a mismatch between
the ->nports counter and registered ports. This patch modifies the counting
strategy, to make ->nports represent the number of registered ports instead
of the number of available childs.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>