VMCI: Fix broken context ID retrieval

I'm an idiot.  The context ID can be a really large unsigned number, which
means it'll appear negative as an int.  So actually the right fix here is just
to set it regardless of the returned value (but only for this particular
hypercall; normally we would check it).

Acked-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Andy King <acking@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>