net: mvpp2: simplify maintaining enabled ports' list

'port_count' field of the mvpp2 structure holds an overall amount
of available ports, based on DT nodes status. In order to be prepared
to support other HW description, obtain the value by incrementing it
upon each successful port initialization. This allowed for simplifying
port indexing in the controller's private array, whose size is now not
dynamically allocated, but fixed to MVPP2_MAX_PORTS.

This patch simplifies creating and filling list of enabled ports and
is a part of the preparation for adding ACPI support in the mvpp2 driver.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>