i40e: implement DCB support infastructure

Intel XL710 series of adapters support QoS as per the
IEEE 802.1 DCB (Data Center Bridging) standard.
This is supported in conjuction with:
- Enhanced Transmission Selection (ETS) - IEEE 802.1Qaz
- Priority Flow Control (PFC) - IEEE 802.1Qbb
- DCB eXchange Protocol (DCBX) - IEEE 802.1Qaz

On Intel XL710 adapters DCBX is performed by the adapter
firmware. The firmware runs DCBX in willing mode and configures
the port as per the DCB settings recommended by it's link
partner.

By default in absence of any DCBX; firmware would configure the
port with a single traffic class and all of the port bandwith
will be allocated to that traffic class.

This patch adds functions and calls to support querying and
configuring DCB using firmware APIs.

Signed-off-by: Neerav Parikh <Neerav.Parikh@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-By: Jack <Morgan&lt;jack.morgan@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>