IB/mlx5: Enable WQ creation and modification with cvlan offload

Allow creating a WQ with cvlan stripping considering device's
capabilities. The default value was fixed to disable vlan stripping
till was asked explicitly.

In addition, allow modification of a WQ to turn on/off this property.

Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>