bnxt_en: don't try to offload VLAN 'modify' action

bnxt offload code currently supports only 'push' and 'pop' operation: let
.ndo_setup_tc() return -EOPNOTSUPP if VLAN 'modify' action is configured.

Fixes: 2ae7408f ("bnxt_en: bnxt: add TC flower filter offload support")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Sathya Perla <sathya.perla@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>