net/nfp: fix disabling VLAN stripping

net/nfp: fix disabling VLAN stripping

"vlan set strip off 0" can't work, due to the incorrect usage
of the mask, it just represent that the status is changed or not,
not represent offloaded or not, so that the driver send the
error control message to the nic.

Now, by first inspect the mask of things that changed, and then
change the requested state if VLAN stripping according the
requested offload status. So this change can fix this bug.

Fixes: d4a27a3b092a ("nfp: add basic features")
Cc: stable@dpdk.org
Signed-off-by: Peng Zhang <peng.zhang@corigine.com>
Signed-off-by: Yong Xu <yong.xu@corigine.com>
Signed-off-by: Chaoyong He <chaoyong.he@corigine.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@corigine.com>