net: dsa: sja1105: remove sp->dp

It looks like this field was never used since its introduction in commit
227d07a0 ("net: dsa: sja1105: Add support for traffic through
standalone ports") remove it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>