net/mlx5e: Make sure fib dev exists in fib event

For unreachable route entry the fib dev does not exists.

Fixes: 8914add2

 ("net/mlx5e: Handle FIB events to update tunnel endpoint device")
Reported-by: Dennis Afanasev <dennis.afanasev@stateless.net>
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>