net: dsa: get ports within parsing code

There is no point into hiding the -EINVAL error code in ERR_PTR from a
dsa_get_ports function, simply get the "ports" node directly from within
the dsa_parse_ports_dn function.

This also has the effect to make the pdata and device tree handling code
symmetrical inside _dsa_register_switch.

At the same time, rename dsa_parse_ports_dn to dsa_parse_ports_of
because _of is a more common suffix for device tree parsing functions.

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>