net/i40e: remove compiler barrier from NEON Rx

net/i40e: remove compiler barrier from NEON Rx

As packet length extraction code was simplified,the ordering
was not necessary any more.[1]

2% performance gain was measured on Marvell ThunderX2.
4.3% performance gain was measured on Ampere eMAG80

[1] http://mails.dpdk.org/archives/dev/2016-April/037529.html

Fixes: ae0eb310f253 ("net/i40e: implement vector PMD for ARM")
Cc: stable@dpdk.org
Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>