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

Null checks in dgap_tty_digisetcustombaud() are already done by
dgap_tty_ioctl() and change "tty" as a paramter of this function
to "ch", "bd and "un".

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