net: dsa: Fix dependencies on bridge

DSA now uses one of the symbols exported by the bridge,
br_vlan_enabled(). This has a stub, if the bridge is not
enabled. However, if the bridge is enabled, we cannot have DSA built
in and the bridge as a module, otherwise we get undefined symbols at
link time:

   net/dsa/port.o: In function `dsa_port_vlan_add':
   net/dsa/port.c:255: undefined reference to `br_vlan_enabled'
   net/dsa/port.o: In function `dsa_port_vlan_del':
   net/dsa/port.c:270: undefined reference to `br_vlan_enabled'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>