dpaa2-switch: unify the FDB update logic in dpaa2_switch_port_set_fdb()

dpaa2-switch: unify the FDB update logic in dpaa2_switch_port_set_fdb()

For both the join and leave paths, the logic goes through the following
steps: determines which FDB should be used on a port after the current
changeupper change, populate the private port structures with the new
FDB and, if necessary, make as not used the old FDB.
Instead of having two distinct paths inside the
dpaa2_switch_port_set_fdb() for linking=true and linking=false, unify
them. This will hopefully help in making this function easier to read.

No behavior changes are expected.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://patch.msgid.link/20260610150912.1788482-6-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>