staging: dgnc: free memory allocated

The memory allocated in dgnc_tty_register() for two objects is not freed
anywhere.This patch addresses this by freeing the memory in
dgnc_tty_uninit.

Signed-off-by: Hari Prasath Gujulan Elango <hgujulan@visteon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>