V4L/DVB (13009): gspca - stv06xx-hdcs: Reduce exposure range

Due to rounding and clipping, exposure and gain settings do not map to
unique register values.  Rather than read the registers and report gain
and exposure that may be different than the values that were set, just
cache the latest values that were set and report them.  Reduce exposure
range from 0-65535 to 0-255 so libv4l's autogain doesn't take forever.
Remove vestiges of driver signal processing that is now handled by
libv4l.

Signed-off-by: James Blanford <jhblanford@gmail.com>
Signed-off-by: Erik Andrén <erik.andren@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>