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

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>