net/mlx5: fix input register for ASO object

net/mlx5: fix input register for ASO object

Existing code uses the hard-coded value REG_C_5 as input for function
mlx5dv_dr_action_create_flow_hit().

This patch updates function mlx5_flow_get_reg_id() to return the
selected REG_C value for ASO Flow Hit operation.
The returned value is used, after reducing offset REG_C_0, as input
for function mlx5dv_dr_action_create_flow_hit().

Fixes: f935ed4b645a ("net/mlx5: support flow hit action for aging")

Signed-off-by: Dekel Peled <dekelp@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>