dpaa2-switch: move FDB selection for join path into a helper

dpaa2-switch: move FDB selection for join path into a helper

The dpaa2_switch_port_set_fdb() function handles the setup of the FDB
for both changeupper cases: join and leave. Move the code block which
handles the join path into a new helper - dpaa2_switch_fdb_for_join() -
with the hope that the entire function will become easier to read and
extend with other use cases in the future.

This new helper just determines and returns what FDB should be used for
a specific port, the cleanup of the old FDB and the actual setup in the
per port structure remains in the dpaa2_switch_port_set_fdb() function.

No changes in the actual behavior are intended.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://patch.msgid.link/20260610150912.1788482-4-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>