media: ov5670: Fix probe on ACPI

devm_clk_get() will return either an error or NULL, which the driver
handles, continuing to use the clock of reading the value of the
clock-frequency property.

However, the value of ov5670->xvclk is left as-is and the other clock
framework functions aren't capable of handling error values.

Use devm_clk_get_optional() to obtain NULL instead of -ENOENT.

Fixes: 8004c91e

 ("media: i2c: ov5670: Use common clock framework")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>