fixup dsa-check-running-kernel again after ubuntu changed their packages

Now ubuntu's linux-image-generic package has
    Depends: linux-image-4.4.0-143-generic | linux-image-unsigned-4.4.0-143-generic
which caused the previous code to consider that as a single package
named "linux-image-4.4.0-143-generic|linux-image-unsigned-4.4.0-143-generic".

Let's just ignore the alternative and take the first one, since it works
for us that way anyway.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>