net/mlx5e: Move counter creation call to alloc_flow_attr_counter()

Move shared code to alloc_flow_attr_counter() for reuse by the next patches.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>