staging: dgnc: remove useless switch-case statements

The dgnc_tty_send_break() has a switch-case condition for msec.
It is no use except case -1.

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