[CORE] netswitch: Call completion wait only when xfer list is empty

Always calling completion wait before checking xfer list is not
wrong but adds unnecassy overhead if we have continuous traffic
hence, we always check xfer list first and do completion wait
only if xfer list is empty.

This simple change does improve network performance little bit.

Signed-off-by: Anup Patel <anup@brainfault.org>