net: mvpp2: fix port list indexing

The private port_list array has a list of pointers to mvpp2_port
instances. This list is allocated given the number of ports enabled in
the device tree, but the pointers are set using the port-id property. If
on a single port is enabled, the port_list array will be of size 1, but
when registering the port, if its id is not 0 the driver will crash.
Other crashes were encountered in various situations.

This fixes the issue by using an index not equal to the value of the
port-id property.

Fixes: 3f518509

 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Yan Markman <ymarkman@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>