media: v4l2-ioctl: rename a temp var that stores _IOC_SIZE(cmd)

Instead of just calling it as "size", let's name it as "ioc_size",
as it reflects better its contents.

As this is constant along the function, also mark it as const.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>