bnxt_en: allow firmware to disable VLAN offloads

Bare-metal use cases require giving firmware and the embedded
application processor control over VLAN offloads. The driver should
not attempt to override or utilize this feature in such scenarios
since it will not work as expected.

Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>