MFC: 279078

MFC: 279078

sfxge: add assertions that required event handlers are implemented

efx_ev_mcdi() does not assert or check that all event handlers it
calls are non-null.  Add assertions at the top for all required
event handlers, as some events (in the case of this bug, monitor
events) are rare.

Submitted by:   Ben Hutchings
Sponsored by:   Solarflare Communications, Inc.
Approved by:    gnn (mentor)