staging: dgnc: Fix a NULL pointer dereference

The error handling for print_drive after calling tty_alloc_driver()
was needed. But there was error handling but too late for this.
Error handling code moved after tty_alloc_driver() call.

Fixes: 60b3109e ("staging: dgnc: use tty_alloc_driver instead of kcalloc")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>