media: ipu3-cio2: rename ipu3-cio2-main.c back to ipu3-cio2.c

cio2 bridge driver is moved out of ipu3, current ipu3 cio2 driver only has
1 source file, then we can rename the source file back to ipu3-cio2.c.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>