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 <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>