net: lan966x: check for ptp to be enabled in lan966x_ptp_deinit()

If ptp was not enabled due to missing IRQ for instance,
lan966x_ptp_deinit() will dereference NULL pointers.

Fixes: d0964594

 ("net: lan966x: Add support for ptp clocks")
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>