nvmet: expose firmware revision to configfs

Allow user to set currently active firmware revision

Reviewed-by: Konstantin Shelekhin <k.shelekhin@yadro.com>
Reviewed-by: Dmitriy Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Aleksandr Miloserdov <a.miloserdov@yadro.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>