igb: Add switch case for supported hardware to igb_ptp_remove.

PTP initialization is only done on supported parts, so remove needs
same checks or it will cause crashes on systems with igb devices that
don't support PTP.  This patch adds those checks to the exit function.

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>