sfxge(4): prepare sfxge to be RSS API aware

This change is needed because 'opt_rss.h' is included by multiple source
files and RSS macro is defined as 1 within the file during build process
if option RSS is enabled in the kernel.

Submitted by: Ivan Malov <Ivan.Malov at oktetlabs.ru>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D6718