camera: vfe32: Use vfe subdev for NOTIFY_VFE_IRQ notification.

By doing this we don't need to look up mctl instance, which is
not necessary because NOTIFY_VFE_IRQ does nothing more than
routing IRQ to VFE subdev.

If we were to look up mctl instance, for IRQs like RESET_ACK,
we won't know whether we should use PIX or RDI instance.

Change-Id: Idbecc47d7e031302cc4c03aa9936584ea10225d3
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>