igb: Support the get_ts_info ethtool method.

Based on original patch from Richard Cochran <richardcochran@gmail.com>

Original patch caused build errors without CONFIG_IGB_1588_CLOCK and
CONFIG_PPS enabled, since the added code was not properly wrapped.

CC: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>