MFC r310750

sfxge(4): add missing barrier in common code MCDI response handling

The semantics of the MCDI interfacve require reading the first
dword of the header before any other data in the buffer. Add
a barrier to the common code MCDI handler to enforce this.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.