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

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

Move the FDB selection for when a port leaves bridge into a new helper -
dpaa2_switch_fdb_for_leave(). This will hopefully make the
dpaa2_switch_port_set_fdb() function easier to read and follow. The new
helper only determines the FDB to be used, any updates into the private
port structure still gets done in the 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/20260610150912.1788482-5-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>