Staging: hv: Change the signature for vmbus_child_driver_unregister

In preperation for moving the element driver from the
struct driver_context to struct hv_driver, change the
signature for the function vmbus_child_driver_unregister()
to take a pointer to struct device_driver.

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