sfxge(4): add advanced function to extract FW version

Some libefx-based drivers might need this functionality to
indicate DPCPU FW IDs as part of FW version info to assist
experienced users.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18065