sfxge(4): provide simple access to RxQ state in EvQ

[arybchik]

Summary

Packed stream Rx datapath requires access to packed stream state
stored in event queue. Number of credits is upstead in event handler
on a new buffer, packets parsing on 64k boundary crossing and
Rx doorbell push to give credits back.

Sponsored by: Solarflare Communications, Inc.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine