mlxsw: spectrum_ptp: Enable/disable PTP shaper on a port when getting HWTSTAMP on/off

In order to get more accurate hardware time stamping, the driver needs to
enable PTP shaper on the port, for speeds lower than 40 Gbps.

Enable the PTP shaper on the port when the user turns on the hardware
time stamping, and disable it when the user turns off the hardware time
stamping.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>