ena: Add macros which represent LLQ header size in bytes

This commit adds macros which define LLQ entry's
header part size in bytes.

The header part size is calculated by

(LLQ_ENTRY_SIZE - DESC_NUMBER * DESC_LEN)

While DESC_LEN is constant (size of a struct) the other two variables
are received from device and can theoretically change.
For example:

• LLQ entry size can be either 128 or 256
• number of descriptors can vary from 1 to 8

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