igb: Pull adapter out of main path in igb_xmit_frame_ring

We only need the adapter pointer in the case of ptp.  As such we can pull the
adapter out of the main path and place it inside the if statement to avoid
the temptation of accessing the adapter pointer in the fast path.

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>