sfxge: added fallbacks for pre 4.2.1 firmware support

Driver must be able to start against older firmware that is missing
recently added MCDI calls, otherwise firmware upgrade will not be
possible.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D3145