net/mlx5e: Lag, Fix fib_info pointer assignment

Referenced change incorrectly sets single path fib_info even when LAG is
not active. Fix it by moving call to mlx5_lag_fib_set() into conditional
that verifies LAG state.

Fixes: ad11c4f1 ("net/mlx5e: Lag, Only handle events from highest priority multipath entry")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>