cnic: remove use of VLAN_TAG_PRESENT

This just removes VLAN_TAG_PRESENT use.  VLAN TCI=0 special meaning is
deeply embedded in the driver code and so is left as is.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>