sfxge(4): add per-command timeout reporting to the common code

In newer firmware that supports multithreaded MCDI processing,
longer running commands may be run ina background thread. Add
support for drivers to query the appropriate timeout for each
MCDI request.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8968