sfxge(4): request info about outer frame in Rx events

For encapsulated packets, the firmware gives info about the inner frame
fields by default. When not using encapsulation offload, ask for info
about the outer frame instead.

On SFN8xxx with firmware version before v6.4.2.1007 driver reload is
needed after switching from full-feature to low-latency firmware
variant since the driver still thinks that firmware supports
encapsulation, but firmware does not tolerate request to provide info
about outer frame in Rx events.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18105