ltq-atm/ltq-ptm: avoid unnecessary build dependencies

ltq-atm/ltq-ptm: avoid unnecessary build dependencies

Right now, both ltq-adsl-mei and ltq-vdsl-mei are always built, even
when they aren't necessary for the selected variant. This can cause the
build to fail, for example ltq-vdsl-mei doesn't build successfully here
on xway target due to the vectoring callback.

Make these dependencies conditional on the specific package variants,
so they are only built when actually needed.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>