igb: Fix code comments and whitespace

Aligns the multi-line code comments with the desired style for the
networking tree.  Also cleaned up whitespace issues found during the
cleanup of code comments (i.e. remove unnecessary blank lines,
use tabs where possible, properly wrap lines and keep strings on a
single line)

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>