staging: dgrp: check return value of alloc_tty_driver

alloc_tty_driver was always assumed to succeed.  Add code to check the
return value and return -ENOMEM if alloc_tty_driver fails.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>