Allow usage of more RX descriptors than 1 in ENA driver

Using only 1 descriptor on RX could be an issue, if system would be low
on resources and could not provide driver with large chunks of
contiguous memory.

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/D12871