net: dsa: fix switch tree list

[ Upstream commit 50c7d2ba9de20f60a2d527ad6928209ef67e4cdd ]

If there are multiple switch trees on the device, only the last one
will be listed, because the arguments of list_add_tail are swapped.

Fixes: 83c0afae ("net: dsa: Add new binding implementation")
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>