media: i2c: ov9640: change array index or length variables to unsigned

The driver uses variables to store frame resolutions and to indexing
various arrays. These should be unsigned.

Signed-off-by: Petr Cvek <petrcvekcz@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>