staging: dgnc: audit goto's in dgnc_tty

TODO file requests fix up of error handling.

Audit dgnc_mgmt.c and fix all return paths to be uniform and inline
with kernel coding style.
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>