make nvem a module again

make nvem a module again

Requested in the forum, as some (misbehaving) mellanox module relies
on loading nvme-core and apparently cannot cope with built-in
modules... meh, but we're probably easier to get to make the change
than the mellanox people ;-)
https://forum.proxmox.com/threads/kernel-5-11.86225/post-412645

Originally set when this was another repo:
https://git.proxmox.com/?p=pve-kernel-jessie.git;a=commitdiff;h=0817f603d0e5bc3165f512eec8860543430cfd2d;hp=60c76fe4f5ab178af78be9ca2ea8baad3b4a4bc9

The Debian bug is resolved since quite a bit, bullseye shouldn't be
affected by this at all anymore.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>