[ena] Specify the unused completion queue MSI-X vector as 0xffffffff

[ena] Specify the unused completion queue MSI-X vector as 0xffffffff

Some versions of the ENA firmware (observed on a c6i.large instance in
eu-west-2) will complain if the completion queue's MSI-X vector field
is left empty, even though the queue configuration specifies that
interrupts are not used.

Work around these firmware versions by passing in what appears to be
the magic "no MSI-X vector" value in this field.

Signed-off-by: Michael Brown <mcb30@ipxe.org>