ixgbe: implement SIOCGHWTSTAMP ioctl

This patch adds support for the new SIOCGHWTSTAMP ioctl, which enables a
process to determine the current timestamp configuration. In order to
implement this, store a copy of the timestamp configuration. In
addition, we can remove the 'int cmd' parameter as the new set_ts_config
function doesn't use it. I also fixed a typo in the function
description.

-v2
* Only save the settings after validating them

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