Staging: hv: vmbus: In vmbus_child_driver_unregister() don't set the bus field to NULL

As part of conforming to the Linux Driver Model, do not set the bus field to
NULL when the driver un-registers.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Abhishek Kane <v-abkane@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>