Fix the igb PTP support to explicitly reject any future flags that
get added to the external timestamp request ioctl.

In order to maintain currently functioning code, this patch accepts all
three current flags. This is because the PTP_RISING_EDGE and
PTP_FALLING_EDGE flags have unclear semantics and each driver seems to
have interpreted them slightly differently.

This HW always time stamps both edges:

  flags                                                 Meaning
  ----------------------------------------------------  --------------------------
  PTP_ENABLE_FEATURE                                    Time stamp both edges
  PTP_ENABLE_FEATURE|PTP_RISING_EDGE                    Time stamp both edges
  PTP_ENABLE_FEATURE|PTP_FALLING_EDGE                   Time stamp both edges
  PTP_ENABLE_FEATURE|PTP_RISING_EDGE|PTP_FALLING_EDGE   Time stamp both edges

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>