media: i2c: ov5670: Fix PIXEL_RATE minimum value

The driver currently reports a single supported value for
V4L2_CID_PIXEL_RATE and initializes the control's minimum value to 0,
which is very risky, as userspace might accidentally use it as divider
when calculating the time duration of a line.

Fix this by using as minimum the only supported value when registering
the control.

Fixes: 5de35c9b ("media: i2c: Add Omnivision OV5670 5M sensor support")
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>