net/bnxt: reduce barriers in NEON vector Rx

net/bnxt: reduce barriers in NEON vector Rx

To read descriptors in expected order, barriers are inserted after each
descriptor read. The excessive use of barriers is unnecessary and could
cause performance drop.

Removed barriers between descriptor reads. And changed counting of valid
packets so as to handle discontinuous valid packets. Because out of
order read could lead to valid descriptors that fetched being
discontinuous.

In VPP L3 routing test, 6% performance gain was observed. The test was
done on a platform with ThunderX2 CPU and Broadcom PS225 NIC.

Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>