[media] Default bt878 contrast value

The default_value for the Bt878 V4L2_CID_CONTRAST control is currently
set to 32768. Internally this gets translated to an analog input
circuit gain of 1.19. However, the default gain should be 1.0. This
patch alters the default value to 27648 which corresponds to a gain of
1.0. It also alters the probe routine so that the correct value is
written on board initialisation.

[mchehab@redhat.com: behavior confirmed via Fusion 878a datasheet]
Signed-off-by: Alan McIvor <alan.mcivor@reveal.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>