staging: dgnc: Remove not needed dgnc_driver_start variable

The dgnc_driver_start variable purpose was to indicate if the driver
'start' routine has been called. Now, because the 'start' routine can
only be called once this variable is not needed thus this commit
removes it.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>