net/mlx5: E-Switch, Use the correct error check on returned pointers

The mlx5 flow-steering API (mlx5_create_flow_table/group/rule) never
returns null pointer on error. Even if it was doing that, checking
for IS_ERR_OR_NULL(p) and then returning PTR_ERR(p) would have cause
bugs, since PTR_ERR(NULL) --> success, crash.

To make things more robust and protect against related future bugs,
convert all IS_ERR_OR_NULL checks on returned values to IS_ERR.

Fixes: 5742df0f ('net/mlx5: E-Switch, Introduce VST vport ingress/egress ACLs')
Fixes: 86d722ad ('net/mlx5: Use flow steering infrastructure for mlx5_en')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>