[media] pxa_camera: move interrupt to tasklet

In preparation for dmaengine conversion, move the camera interrupt
handling into a tasklet. This won't change the global flow, as this
interrupt is only used to detect the end of frame and activate DMA fifos
handling.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>