sfxge(4): allow to request inner classes for Rx packets

If HW/FW supports tunnel encapsulations, Rx event may contain
either inner or outer packet classes. By default outer classes
are requested. Make it possible to request inner classes to
have more information about packet type and allow to interpret
inner frame checksum validation results correctly.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18133