Staging: hv: remove struct vmbus_driver

It's only a wrapper for the struct hv_driver structure, so just use that
instead, as there are no other fields left in it at the moment.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>