igb: Split Rx timestamping into two separate functions

In order to support page based receive we will need to split up the two
different types of timestamping into two separate functions.  The first one
will handle legacy timestamps with the value in the register, and the new
one will handle timestamps in the Rx buffer itself.

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