net: dsa: only check presence of link property

When parsing a port, simply use of_property_read_bool which checks the
presence of a given property, instead of parsing the link phandle.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>