V4L/DVB (3449): Cx88 default picture controls values

V4L/DVB (3449): Cx88 default picture controls values

This patch fixes default values for some picture controls:
 - brightness set to 50% by default (now is 0%)
 - hue set to 50% by default (now is 0%)
 - sets saturation to datasheet value
 - volume set to 0dB (now is -32dB)
and some left small fixes:
 - twice offset adding
 - balance didn't follow datasheet (bits[0:5] = attenuation;
   bit[6] = channel to provide attenuation)

Signed-off-by: Marcin Rudowski <mar_rud@poczta.onet.pl>
Signed-off-by: Ian Pickworth <ian@pickworth.me.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>