e1000e: apply burst mode settings only on default

Devices that support FLAG2_DMA_BURST have different default values
for RDTR and RADV. Apply burst mode default settings only when no
explicit value was passed at module load.

The RDTR default is zero. If the module is loaded for low latency
operation with RxIntDelay=0, do not override this value with a burst
default of 32.

Move the decision to apply burst values earlier, where explicitly
initialized module variables can be distinguished from defaults.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-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>