net: dsa: Move ports assignment closer to error checking

Move the assignment of ports in _dsa_register_switch() closer to where
it is checked, no functional change. Re-order declarations to be
preserve the inverted christmas tree style.

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