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

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.
Differential Revision: https://reviews.freebsd.org/D18099