V4L/DVB: gspca - sn9c20x: Fix exposure control for HV7131R sensor

Make the range of exposure values (0-0x1770) distribute evenly through
HV7131R's exposure control bytes.

Signed-off-by: German Galkin <galkinga@gmail.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>