i40e/i40evf: Add a workaround to drop all flow control frames

This patch adds a workaround to drop any flow control frames from being
transmitted from any VSI. FW can still send flow control frames if flow
control is enabled.

With this patch in place a malicious VF cannot send flow control or PFC
packets out on the wire.

Change-ID: I4303b24e98b93066d2767fec24dfe78be591c277
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>