net/mlx5e: Make function mlx5i_grp_sw_update_stats() static

Fixes the following sparse warning:

drivers/net/ethernet/mellanox/mlx5/core/ipoib/ipoib.c:119:6: warning:
 symbol 'mlx5i_grp_sw_update_stats' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>