staging: dgap: introduce dgap_free_irq()

dgap_free_irq() will free the irq which is requested in
dgap_request_irq().

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