media: ov772x: omit consumer ID when getting clock reference

Currently the ov772x driver obtains a clock with a specific consumer ID.
As there's a single clock for this driver, we could omit clock-names
property in device tree by passing NULL as a consumer ID to clk_get().

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>