ptp: support event queue reader channel masks

On systems with multiple timestamp event channels, some readers might
want to receive only a subset of those channels.

Add the necessary modifications to support timestamp event channel
filtering, including two IOCTL operations:

- Clear all channels
- Enable one channel

The mask modification operations will be applied exclusively on the
event queue assigned to the file descriptor used on the IOCTL operation,
so the typical procedure to have a reader receiving only a subset of the
enabled channels would be:

- Open device file
- ioctl: clear all channels
- ioctl: enable one channel
- start reading

Calling the enable one channel ioctl more than once will result in
multiple enabled channels.

Signed-off-by: Xabier Marquiegui <reibax@gmail.com>
Suggested-by: Richard Cochran <richardcochran@gmail.com>
Suggested-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>