net:dsa: check for EPROBE_DEFER from dsa_dst_parse()

Since there can be multiple dsa switches stacked together but
not all of devicetree nodes available at the time of calling
dsa_dst_parse(), EPROBE_DEFER can be returned by it. When this
happens, only the last dsa switch has to be deleted by
dsa_dst_del_ds(), but not the whole list, because next time linux
cames back to this function it will try to add only the last dsa
switch which returned EPROBE_DEFER.

Signed-off-by: Volodymyr Bendiuga <volodymyr.bendiuga@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>