staging: dgnc: implement proper error handling in dgnc_start()

dgnc_start() ignores errors in class_create() and device_create()
and it does not deallocate resources if dgnc_tty_preinit() fails.

The patch implements proper error handling.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>