net: bridge: fix direct access to bridge vlan_enabled and use helper

net: bridge: fix direct access to bridge vlan_enabled and use helper

We need to use br_vlan_enabled() helper otherwise we'll break builds
without bridge vlans:
net/bridge//br_if.c: In function ‘br_mtu’:
net/bridge//br_if.c:458:8: error: ‘const struct net_bridge’ has no
member named ‘vlan_enabled’
  if (br->vlan_enabled)
        ^
net/bridge//br_if.c:462:1: warning: control reaches end of non-void
function [-Wreturn-type]
 }
 ^
scripts/Makefile.build:324: recipe for target 'net/bridge//br_if.o'
failed

Fixes: 419d14af9e07 ("bridge: Allow max MTU when multiple VLANs present")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>