net: gptp: Avoid memcpy to same buffer

Do not try to memcpy() the same buffer to itself.

This one also reverts commit 112ecb72
("net: gptp: Fix for coverity CIDs 203471 and 203464") as that
did not fully fix the issue.

Coverity-CID: 203464
Coverity-CID: 203471
Fixes #18394

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>