Avoid repeated calls to dsa_to_port() (some hidden behind dsa_is_user_port
and some in plain sight) by keeping two struct dsa_port references: one
to the port passed as argument, and another to the other ports of the
switch that we're iterating over.

dsa_to_port(ds, i) gets replaced by other_dp, i gets replaced by
other_port which is derived from other_dp->index, dsa_is_user_port is
handled by the DSA iterator.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>