examples/l3fwd-power: fix Rx without interrupt

examples/l3fwd-power: fix Rx without interrupt

This existing code cause the platform to start receiving packet
immediately irrespective of interrupts available or not.
If the platform does not support Rx interrupt, it shall not start
receiving packets immediately. It shall let the timer management work.

Fixes: aee3bc79cc34 ("examples/l3fwd-power: enable one-shot Rx interrupt and polling switch")
Cc: stable@dpdk.org
Signed-off-by: Nikhil Agarwal <nikhil.agarwal@linaro.org>
Acked-by: David Hunt <david.hunt@intel.com>