sfxge(4): define a handle to denote default RSS context

Make the existing filter-specific define more general.
This is the same as MC_CMD_RSS_CONTEXT_ALLOC_OUT_RSS_CONTEXT_ID_INVALID.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18075