igb: Tidy up wrapping for CONFIG_IGB_PTP.

For users without CONFIG_IGB_PTP=y, we should not be compiling any PTP
code into the driver. Tidy up the wrapping in igb to support this.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>