Add support for the LLQv2 and WC in ENA

LLQ (Low Latency Queue) is the feature, that allows pushing header
directly to the device through PCI before even DMA is triggered.

It reduces latency, because device can start preparing packet before
payload is sent through DMA.

To speed up sending data through PCI, the Write Combining is enabled,
which allows hardware to buffer data before sending them on the PCI - it
allows to reduce number of PCI IO operations.

ENAv2 is using special descriptor for the negotiation of the LLQ.
Currently, only the default configuration is supported.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.