media: usb: make i2c_adapter const

Make these const as they are only used in a copy operation.
Done using Coccinelle

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Mike Isely <isely@pobox.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>