igb: Fix warning pin may be used uninitialized

When building the kernel using the gcc 4.8.3 compiler included in Fedora 20
I was repeatedly seeing the warning:

 drivers/net/ethernet/intel/igb/igb_ptp.c: In function ‘igb_ptp_feature_enable_i210’:
 drivers/net/ethernet/intel/igb/igb_ptp.c:395:21: warning: ‘pin’ may be used uninitialized in this function
 [-Wmaybe-uninitialized]
   tssdp &= ~ts_sdp_en[pin];
                     ^
 drivers/net/ethernet/intel/igb/igb_ptp.c:471:6: note: ‘pin’ was declared here
   int pin;
       ^

To resolve it I am assigning the pin a value of -1 when it is instantiated.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>