net/mlx5e: Use function pointers for RX data path handling

In preparation for Striding RQ feature, which will need its own
RX handlers.
This patch does not change any functionality.

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