V4L/DVB (9562): gspca: Set the default frame rate to 30 fps for sensor ov764x in ov519.

This patch fixes the bad picture at low resolution (320x240).

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