port_index is used an index into an array, and this information comes
from Device Tree, make sure that port_index is not equal to the array
size before using it. Move the check against port_index earlier in the
loop.

Fixes: 5e95329b: ("dsa: add device tree bindings to register DSA switches")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>