net/mlx5: Fix unused function warning of mlx5i_flow_type_mask

The cited commit is causing unused-function warning[1] when
CONFIG_MLX5_EN_RXNFC is not set.
Fix this by moving the function into the ifdef, where it's only used

[1]
warning: ‘mlx5i_flow_type_mask’ defined but not used [-Wunused-function]

Fixes: 9fbe1c25 ("net/mlx5i: Enable Rx steering for IPoIB via ethtool")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>