ixgbe: PTP get_ts_info missing software support

This patch corrects the ethtool get_ts_info functon which did not state that
software timestamping was supported, even though it is.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
CC: Stable <stable@vger.kernel.org> [3.5]
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>