Staging: hv: Use generic device_driver shutdown function

In preparation for moving all the state from struct driver_context
to struct hv_driver, eliminate the shutdown() function from
struct driver_context and use generic device_driver shutdown()
function.

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>