Staging: hv: make some vmbus_drv functions static

Now that vmbus_drv.c is merged with vmbus.c, some of the newly global
functions can now be marked static.

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