net: dsa: make tree index unsigned

Similarly to a DSA switch and port, rename the tree index from "tree" to
"index" and make it an unsigned int because it isn't supposed to be less
than 0.

u32 is an OF specific data used to retrieve the value and has no need to
be propagated up to the tree index.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>