gpio: grgpio: off by one in grgpio_to_irq()

"gc->ngpio" is a number between 1 and GRGPIO_MAX_NGPIO.  If "offset" is
GRGPIO_MAX_NGPIO then we're going one step beyond the end of the
priv->lirqs[] array.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>