net/mlx5: fix detection and error for multiple item layers

net/mlx5: fix detection and error for multiple item layers

1. The check for the Eth item was wrong. causing an error with
flow rules like:

flow create 0 ingress pattern eth / vlan vid is 13 / ipv4 / gre / eth /
vlan vid is 15 / end actions drop / end

2. align all error messages.

3. align multiple item layers check.

Fixes: 23c1d42c7138 ("net/mlx5: split flow validation to dedicated function")
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>