net: dsa: sja1105: register switch before assigning port private data

Like the dsa_switch_tree structures, the dsa_port structures will be
allocated on switch registration.

The SJA1105 driver is the only one accessing the dsa_port structure
after the switch allocation and before the switch registration.
For that reason, move switch registration prior to assigning the priv
member of the dsa_port structures.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>