bnxt_en: clean up VLAN feature bit handling

The hardware VLAN offload feature on our NIC does not have separate
knobs for handling customer and service tags on RX. Either offloading
of both must be enabled or both must be disabled. Introduce definitions
for the combined feature set in order to clean up the code and make
this constraint more clear. Technically these features can be separately
enabled on TX, however, since the default is to turn both on, the
combined TX feature set is also introduced for code consistency.

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>