cxgb4: configure HW VLAN extraction through FW

HW VLAN extraction needs to be configured through FW to work correctly in
virtualization environments.  Remove the direct register manipulation and
rely on FW.

Signed-off-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>