jack: Don't fail module-jackdbus-detect loading if the channels argument is not given.

jack: Don't fail module-jackdbus-detect loading if the channels argument is not given.

The u->channels <= 0 check failed if the channels argument was not
given at all, making the whole module loading fail. I don't think the
check is necessary at all - negative values are not possible, and if
someone gives 0 as the argument, it's probably ok if we act as if
there was no channels argument at all.