Drivers:hv: Fix a bug in vmbus_driver_unregister()

The function vmbus_exists() was introduced recently to deal with cases where
the vmbus driver failed to initialize and yet other Hyper-V drivers attempted
to register with the vmbus bus driver. This patch introduced a bug where
vmbus_driver_unregister() would fail to unregister the driver. This patch
fixes the problem.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Fuzhou Chen <fuzhouch@microsoft.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>