staging: dgap: removes redundant null check and change the paramter for dgap_param()

The dgap_param() has a paramter which is tty_struct and
use variables in that struct. That variables which are "ch", "bd", "bs"
and "un" do not need to check NULL so these statements are removed.

And also change the parameter of this function because
it is possible to let someone know what paramters
are needed for this function.

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