mlxsw: spectrum: Forbid linking of VLAN devices to devices that have uppers

Jiri Slaby noticed that the backport of upstream commit 25cc72a33835
("mlxsw: spectrum: Forbid linking to devices that have uppers") to
kernel 4.9.y introduced the same check twice in the same function
instead of in two different places.

Fix this by relocating one of the checks to its intended place, thus
preventing unsupported configurations as described in the original
commit.

Fixes: 73ee5a73 ("mlxsw: spectrum: Forbid linking to devices that have uppers")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>