net: mscc: ocelot: make ocelot_init_timestamp take a const struct ptp_clock_info

It is a good measure to ensure correctness if the structures that are
meant to remain constant are only processed by functions that thake
constant arguments.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>