Fix install of CRAS ALSA plugin.

`install -D` only works as intended if the destination is the full filename (it only creates parent directories of the destination).
Since we are installing to a directory, it only creates its parents and not the directory itself...

This patch removes `-D`, and calls `mkdir -p` beforehand.

Fixes issue reported by @DennisLfromGA in https://github.com/dnschneid/crouton/pull/350#issuecomment-25183341 .

At some point we should find a way to automatically install fresh chroots of all our supported releases to catch these simple bugs...