staging: dgnc: ch->ch_bd is already assigned to bd variable

The bd variables in functions are already assigned from
ch->ch_bd but it is not used in those functions except checking NULL.

The ch->ch_bd could be replaced with bd variable.

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