net: mscc: ocelot: convert to ocelot_port_txtstamp_request()

Convert to a common ocelot_port_txtstamp_request() for TX timestamp
request handling.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>