examples/l3fwd-power: fix Rx descriptor size

examples/l3fwd-power: fix Rx descriptor size

L3fwd power app monitors the RX queues to see if the polling frequency
should be adjusted (the busier the queue, the higher the frequency).
The app uses several thresholds in the ring to determine the frequency,
being 96 the highest one, when frequency should be highest.

The problem is that the difference between this value and the ring size
is not big enough (128 - 96 = 32 descriptors), which means that
if the descriptors are not replenished quick enough, queue might
not be busy, but the app would think that it is, because 96th descriptor
is set.

Therefore, by increasing this gap (increasing the RX ring size),
we make sure that this false measurement will not happen.

Fixes: b451aa39db31 ("examples/l3fwd-power: use DD bit rather than RX queue count")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>