loader: md: Use default func for fmtdev and parsedev

The default function are enough for md so use them instead of the
disks ones that doesn't work for it anymore.

Reviewed by: imp
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: now
Differential Revision: https://reviews.freebsd.org/D38218

(cherry picked from commit 04afa8cc370e6bb7302b6fe09c8d27a606fe414e)