net: dsa: get master device at port parsing time

Fetching the master device can be done directly when a port is parsed
from device tree or pdata, instead of waiting until dsa_dst_parse.

Now that -EPROBE_DEFER is returned before we add the switch to the tree,
there is no need to check for this error after dsa_dst_parse.

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>