bnx2x: Fix VLANs null-pointer for 57710, 57711

Commit 05cc5a39

 "bnx2x: add vlan filtering offload" introduced
a regression in regard for vlans for 57710, 57711 adapters -
Loading 8021q module on a machine with such an adapter would cause
a null pointer dereference, as the driver mistakenly publishes it
has capabilities for vlan CTAG filtering.

Reported-by: Otto Sabart <osabart@redhat.com>
Signed-off-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: Ariel Elior <Ariel.Elior@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>