mfd: max8925: Fix onkey driver irq base

update onkey driver's irq base, it should get from max8925,
but not save in a private value

Signed-off-by: Qing Xu <qingx@marvell.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>