staging: dgap: remove redundant setting a variable

The brd(board_t) is initialized with zero, so "intr_used"
is not needed to set zero when request_irq() is failed.

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