igb: Fix sparse warning in igb_ptp_rx_pktstamp

This change fixes a sparse warning triggered by us casting the timestamp in
the packet as a u64 instead of as a __le64.  This change corrects that in
order to resolve the sparse warning.

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