dpdk: fix prefetch assert on Arm

dpdk: fix prefetch assert on Arm

  CLIB_PREFETCH (cop[1], CLIB_CACHE_LINE_BYTES * 3, STORE);

Note on 64 bytes cache line size arm machines,
CLIB_CACHE_LINE_BYTES 128
CLIB_CACHE_PREFETCH_BYTES 6

above CLIB_PREFETCH () macro will be expand to
ASSERT ((size) <= 4 * CLIB_CACHE_PREFETCH_BYTES);
it will hit assert due to size (i.e. 3 * 128) > 4 * 64

Solution:
Change to CLIB_PREFETCH (cop[1], sizeof(*cop[1]), STORE);

Type: fix
Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Change-Id: Id0981fd5bd2b25ff71db4197b25578d0b7a9803e