staging: dgnc: remove useless variable 'intr_tx'

The 'intr_tx' variable was used only for increasing.
So the 'intr_tx' 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>