staging: dgnc: remove unused variable 'intr_modem'

The 'intr_modem' variable was used only for increasing.
So the 'intr_modem' variable is not useful for this driver.

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