ptp: Fix possible memory leak caused by invalid cast

Fixes possible leak of PTP virtual clocks.

The number of PTP virtual clocks to be unregistered is passed as
'u32', but the function that unregister the devices handles that as
'u8'.

Fixes: 73f37068 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>