staging: dgnc: remove useless variables

The dgnc_major_serial_registered and dgnc_major_serial_registered
do not need to use to check whether the tty driver is registered or not.
These variables are used only in dgnc_cleanup_tty() function,
This function will be called normally with initialized board structure.
It means the dgnc_cleanup_tty() cannot be called with unregistered tty.

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