i40e: Add vf-true-promisc-support priv flag

This patch adds priv-flag knob to configure global true promisc
support. With this patch the user can decide the flavor of
promiscuous that the VFs will see when promiscuous mode is enabled
on the interface. Since this a global setting for the whole device,
the priv-flag is exposed only on the first PF of the device.

The default is true promisc support is off, which means the promisc
mode for the VF will be limited/defport mode.

For the PF, we still will be in limited promisc unless in MFP mode
irrespective of the flavor picked through this knob.

Usage:
On PF0
ethtool --show-priv-flags p261p1
Private flags for p261p1:
MFP                    : off
LinkPolling            : off
flow-director-atr      : on
veb-stats              : off
hw-atr-eviction        : off
vf-true-promisc-support: off

to enable setting true promisc
ethtool --set-priv-flags p261p1 vf-true-promisc-support on

At this point if the VF is set to trust and promisc is enabled
on the VF through
ip link set ... promisc on
The VF/VFs will be able to see ALL ingress traffic

Change-Id: I8fac4b6eb1af9ca77b5376b79c50bdce5055bd94
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>