net: enetc: extract enetc_update_ptp_sync_msg() to handle PTP Sync packets

net: enetc: extract enetc_update_ptp_sync_msg() to handle PTP Sync packets

Move PTP Sync packet processing from enetc_map_tx_buffs() to a new helper
function enetc_update_ptp_sync_msg() to simplify the original function.
Prepare for upcoming ENETC v4 one-step support. There is no functional
change. It is worth mentioning that ENETC_TXBD_TSTAMP is added to replace
0x3fffffff.

Prepare for upcoming ENETC v4 one-step support.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250829050615.1247468-11-wei.fang@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>