usecase: resolve shellcheck warning for L3 forwarding case

Resolve warnings reported by shellcheck for scripts of L3 forwarding
usecase.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: I12963b0d4a1118886e8f4ecc31b5de11621e0515