staging: dgnc: introduce the dgnc_free_irq()

The dgnc_free_irq() will free the requested IRQ from
the dgnc_request_irq().

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