platform/x86: ISST: Reset default callback on unregister

platform/x86: ISST: Reset default callback on unregister

When multiple clients are registered and some of those modules are
removed, the default IOCTL callback for those clients are still not
NULL. Calling them will result in crash.

Set the default IOCTL callback pointer to NULL on unregister.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://lore.kernel.org/r/20230612224033.2382527-2-srinivas.pandruvada@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>