net/mlx5: change pkt burst select function prototype

net/mlx5: change pkt burst select function prototype

Change the function prototype to return the function pointer of the
selected Tx/Rx burst function instead of assigning it directly to the
device context.

Such change will enable to use those select functions to query the burst
function that will be selected according to the device configuration.

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>