VMCI: Fix "always true condition"

vmci_send_datagram() returns an int, with negative values indicating failure.
But we store it locally in a u32, which makes comparison of >= 0 useless.
Fixed to use an int.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andy King <acking@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>