net: dsa: fix filling routing table from OF description

According to description in 'include/net/dsa.h', in cascade switches
configurations where there are more than one interconnected devices,
'rtable' array in 'dsa_chip_data' structure is used to indicate which
port on this switch should be used to send packets to that are destined
for corresponding switch.

However, dsa_of_setup_routing_table() fills 'rtable' with port numbers
of the _target_ switch, but not current one.

This commit removes redundant devicetree parsing and adds needed port
number as a function argument. So dsa_of_setup_routing_table() now just
looks for target switch number by parsing parent of 'link' device node.

To remove possible misunderstandings with the way of determining target
switch number, a corresponding comment was added to the source code and
to the DSA device tree bindings documentation file.

This was tested on a custom board with two Marvell 88E6095 switches with
following corresponding routing tables: { -1, 10 } and { 8, -1 }.

Signed-off-by: Pavel Nakonechny <pavel.nakonechny@skitlab.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/dsa.txt

@@ -19,7 +19,9 @@ the parent DSA node. The maximum number of allowed child nodes is 4
 (DSA_MAX_SWITCHES).
 Each of these switch child nodes should have the following required properties:
 
-- reg			: Describes the switch address on the MII bus
+- reg			: Contains two fields. The first one describes the
+			  address on the MII bus. The second is the switch
+			  number that must be unique in cascaded configurations
 - #address-cells	: Must be 1
 - #size-cells		: Must be 0