bnx2: Fix logic to setup VLAN rx tagging.

We should now be checking BNX2_FLAG_CAN_KEEP_VLAN to determine how
to set the VLAN rx tagging in the RX_MODE register.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>