Staging: hv: Change the signature for vmbus_child_device_register()

In preparation for getting rid of the vmbus_child_dev_add() function,
modify the signature of vmbus_child_device_register(). Note that
the root device is a global variable.

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