sfxge(4): refactor monitors support

Remove obsolete monitor types since Falcon SFN4000 series adapters
no longer supported by libefx.
Rename MCDI monitors to be consistent with YML.
The code may be simplified and generalized since only MCDI monitors
remain.

Submitted by: Martin Harvey <mharvey at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18249