net/dsa/dsa.c: increment chip_index during of_node handling on dsa_of_probe()

Adding more than one chip on device-tree currently causes the probing
routine to always use the first chips data pointer.

Signed-off-by: Fabian Godehardt <fg@emlix.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>