MAINTAINERS: fix file path for Omnvision OV4689

Correct the driver source path specified in the MAINTAINERS file,
which was mistakenly set due to an oversight during the driver's
initial addition.

Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>