net/mlx5e: Extract creation of rep's default flow rule

Cleaning up the flow of the representors' rx initialization, towards
enabling RSS for the representors.

Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>