staging: dgrp: remove use of real_raw and read_cnt in dgrp_input

dgrp_input used real_raw and read_cnt from struct tty_struct.  Those
members have gone away.  Rework the code to not use them.

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>