sfxge(4): restrict the maximum number of RSS channels by the number of RSS buckets

This is done because one has no point to have more channels since they
will be unused.

Submitted by: Ivan Malov <Ivan.Malov at oktetlabs.ru>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D6720