sfxge(4): do not limit driver RSS table to RSS channels max

Specification of entire RSS table in the driver allows to spread traffic
more equally across CPUs/RSS channels if number of RSS channels is not
power of 2.

Reviewed by: philip
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8910