net/ena: rework getting number of available descriptors
net/ena: rework getting number of available descriptors

ena_com API should be preferred for getting number of used/available
descriptors unless extra calculation needs to be performed.

Some helper variables were added for storing values that are later
reused. Moreover, for limiting the value of sent/received packets to
the number of available descriptors, the RTE_MIN is used instead of
if function, which was doing similar thing but was less descriptive.

Signed-off-by: Michal Krawczyk <a91dcbb4df49d8bfe0a9930d7ff911a4292ca1f5@semihalf.com>
Reviewed-by: Igor Chauskin <a8d4b4819272c1df83ebe995beed3305455c3333@amazon.com>
Reviewed-by: Guy Tzalik <2e660552b21bf06b7025c5200bbf74d8c7c74097@amazon.com>

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