net/mlx5: refuse empty VLAN flow specification

net/mlx5: refuse empty VLAN flow specification

Verbs specification doesn't help to distinguish between packets having an
VLAN and those which do not have, this ends by having flow rule which does
not react as the user expects e.g.

 flow create 0 ingress pattern eth / vlan / end action queue index 0 / end
 flow create 0 ingress pattern eth / end action queue index 1 / end

are colliding in Verbs definition as in both rule are matching packets with
or without VLAN.
For this reason, the VLAN specification must not be empty, otherwise the
PMD has to refuse it.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>