[media] gspca - sn9c20x: Set the i2c interface speed

The i2c interface speed was set to 400 Kb/s while it is 100 Kb/s
for most sensors.

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