examples/l3fwd: skip Tx queue drain on first iteration

examples/l3fwd: skip Tx queue drain on first iteration

Initialize prev_tsc to cur_tsc. This avoids running the TX queue drain
in the first iteration of the packet processing loop.

Signed-off-by: Kathleen Capella <kathleen.capella@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>