staging: dgnc: remove redundant NULL checks in

The dgnc_block_til_ready() is only used in dgnc_tty_open().
The unit data(struct un_t) was stored into tty->driver_data in dgnc_tty_open().
And also tty and un were tested about NULL so these variables doesn't
need to check for NULL in dgnc_block_til_ready().

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