VMCI: fix error handling path when registering guest driver

When host capabilities check failed or when we were unable to register doorbell
bitmap we were forgetting to set error code and were returning 0 which would
make upper layers believe that probe was successful.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Acked-by: Andy King <acking@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>