ixgbe: Fix MDD events generated when FCoE+SRIOV are enabled

ixgbe: Fix MDD events generated when FCoE+SRIOV are enabled

Orabug: 23177316

When FCoE is enabled with SR-IOV on the X550 NIC the hardware
generates MDD events.

This patch fixes these by setting the expected values in the
Tx context descriptors for FCoE/FIP frames and adding a flush
after writing the RDLEN register.

Signed-off-by: Neerav Parikh <neerav.parikh@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 8b75451be1fc05b6ee3f9d0eaea0006d60caff89)
Signed-off-by: Brian Maly <brian.maly@oracle.com>