ptp: Change ptp_class to a proper bitmask

Change the definition of PTP_CLASS_L2 to not have any bits overlapping with
the other defined protocol values, allowing the PTP_CLASS_* definitions to
be for simple filtering on packet type.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: David S. Miller <davem@davemloft.net>