ptp: dynamic allocation of PHC char devices

As network adapters supporting PTP are becoming more common, machines with
many NICs suddenly have many PHCs, too. The current limit of eight /dev/ptp*
char devices (and thus, 8 network interfaces with PHC) is insufficient. Let
the ptp driver allocate the char devices dynamically.

Tested with 28 PHCs, removing and re-adding some of them.

Thanks to Ben Hutchings for advice leading to simpler and cleaner patch.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>