staging: dgnc: use tty_alloc_driver instead of kcalloc

The tty_alloc_driver() can allocate memory for ttys and termios.
And also allocated memory will be released easily with
put_tty_driver() call.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>