Staging: dgnc: dgnc_driver: Add a missing call to dgnc_tty_uninit

This function is called on the previous and the next failure branches.
This patch adds the call on the branch where it seems to be missing.

Signed-off-by: Quentin Lambert <lambert.quentin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>