media: i2c: ov7670: Use the devm_clk_get_optional() helper

Use devm_clk_get_optional() instead of hand writing it.
This saves some loC and improves the semantic.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>