sfxge(4): allow firmware to auto-configure event queues on Medford

On Medford, licenses are required to enable RX and event cut through and to
disable RX batching. To avoid the need for the driver to make decisions based on
the licensing state, the MC_CMD_INIT_EVQ has been extended to allow us to leave
the decision to the firmware. If the adapter is licensed for low-latency use,
the firmware will choose the optimal settings for latency, otherwise it will use
the best settings for throughput.

For Huntington we still need to choose the settings ourselves.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6717