staging: dgnc: remove too many traverse pointer

The "ch->ch_bd" is already assined to "bd" but this is only
for checking null or MAGIC number.
in the dgnc_tty_ioctl function, bd can be used for referencing
to board_ops structure.

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