ixgbe: Correct setting of RDRXCTL register for X550* devices

Setting the X550* RDRXCTL register should fall through into X540
and 82599, not 82598.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>