net: dsa: xrs700x: check if partner is same as port in hsr join

Don't assign dp to partner if it's the same port that xrs700x_hsr_join
was called with. The partner port is supposed to be the other port in
the HSR/PRP redundant pair not the same port. This fixes an issue
observed in testing where forwarding between redundant HSR ports on this
switch didn't work depending on the order the ports were added to the
hsr device.

Fixes: bd62e6f5 ("net: dsa: xrs700x: add HSR offloading support")
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>