staging: dgnc: removes redundant null check and change

The dgnc_set_modem_info() used only channel_t variable.
Any other variables were used only for checking NULL.

So fist parameter changed from "tty_struct" to "channel_t" and
useless NULL checks and variables are removed.

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