media: ov5670: Fix probe on ACPI

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: 8004c91e2095 ("media: i2c: ov5670: Use common clock framework")
Signed-off-by: Sakari Ailus <[email protected]>
Reviewed-by: Jacopo Mondi <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>