ena: Support LLQ entry size recommendation from device

This commit adds support for receiving LLQ entry size recommendation
from the device. The driver will use the recommended entry size, unless
the user specifically chooses to use regular or large LLQ entry.

Also added enum ena_llq_header_size_policy_t and llq_plociy field in
order to support the new feature.

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