ena: Add ENA_ADMIN_CDESC_MBZ capability

This is a device-side capability that indicates the
device is setting the MBZ fields on the completion descriptors
to zero before sending the descriptors to the driver. This means
that it's safe for the driver to utilize these fields
for corruption checks.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.