firmware: qemu_fw_cfg: remove sysfs entries explicitly

Explicitly remove the file entries from sysfs before dropping the final
reference for symmetry reasons and for consistency with the rest of the
driver.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211201132528.30025-5-johan@kernel.org


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>