net: gptp: Always use GM PRIO root system id for announce messages

A problem occurred while running PTP on a multi-port target
(RENESAS RZT2M) with two ports enabled. Announce messages on the
switched devices master port always contained local clock information
instead information from received on the slave port from the better
GM clock. Depending on the BMCA config this turned into having more
than one GM in the system.

Sending always the locally stored GM information helped to overcome
this issue.

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