staging: dgnc: remove useless variables for saving tty's

It doesn't need to save major number with variable.
And there are no use of these variables(dgnc_serial_major and
dgnc_transparent_print_major)

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