net: gptp: Use local port identity when forwarding sync messages

As defined in IEEE802.1AS-2020 ch. 10.2.12.2.1, the port identity
of an MDSyncSend structure sent from a port shall be set to the
port identity of the sending port according to and ch. 8.5.2.

This commit replaces the port identity before forwarding a sync.

Fixes #68385

Signed-off-by: Manuel Schappacher <manuel.schappacher@hs-offenburg.de>