staging: unisys: fix up device_destroy

Visorbus_remove_instance calls device_unregister which in turn will call
visorbus_release_busdevice. The function visorbus_release_busdevice was
freeing the visor_device. The code in visorbus_remove_instance was then
trying to reference the visor_device. This patch cleans up the code so
the visor_device is not referenced after the unregister.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>