net/mlx4_en: RX, Add a prefetch command for small L1_CACHE_BYTES

A single cacheline might not contain the packet header for
small L1_CACHE_BYTES values.
Use net_prefetch() as it issues an additional prefetch
in this case.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>