[media] mx2_camera: fix pixel clock polarity configuration

When SOCAM_PCLK_SAMPLE_FALLING, just leave CSICR1_REDGE unset, otherwise we get
the inverted behaviour.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>