[media] media: i.MX27 camera: return IRQ_NONE if no IRQ status bit is set

If active_bufs() list is empty and no IRQ status bit is set
we are probably dealing with a share IRQ. Return IRQ_NONE in
this case.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>