media: i2c: ov5467: Fixup error path to release mutex

"87f3ab94

 media: ov5647: Add basic support for multiple sensor modes."
added a return path ov5647_set_fmt that didn't release the device
mutex that it had claimed.
Release the mutex.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>