firmware: dmi-sysfs: Remove "dmi" directory on module exit

With CONFIG_DEBUG_KOBJECT_RELEASE=y, removing and immediately reloading the
dmi-sysfs module causes the following warning:

  sysfs: cannot create duplicate filename '/firmware/dmi'
  kobject_add_internal failed for dmi with -EEXIST, don't try to register things with the same name in the same directory.

The "dmi" directory stays in sysfs until the dmi_kobj is released, and
DEBUG_KOBJECT_RELEASE delays that.

I don't think we can hit this problem in normal usage because dmi_kobj is
static and nothing outside dmi-sysfs can get a reference to it, so the
only way to delay the "dmi" release is with DEBUG_KOBJECT_RELEASE.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>