Allow partial MSI-x allocation in ENA driver

The situation, where part of the MSI-x was not configured properly, was
not properly handled. Now, the driver reduces number of queues to
reflect number of existing and properly configured MSI-x vectors.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12863