ixgbe: move all Rx DMA control register writes to one central location

This change moves all of the Rx DMA control register writes to one central
location.  This should help to avoid accidentally overwriting existing
settings.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>