mlxsw: spectrum_switchdev: Unify VxLAN leave function

The function mlxsw_sp_bridge_vxlan_leave() is currently split between
VLAN-aware and VLAN-unaware bridges, but actually both types can use the
same function.

The function needs to resolve the FID that corresponds to the VxLAN
device and disable NVE encapsulation on it. Instead of looking up the
FID differently for VLAN-aware and VLAN-unaware bridges, we can always
use the VxLAN's device VNI.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>