media: radio-usb-si4713: make i2c_adapter const

Make this const as it is only used in a copy operation.
Done using Coccinelle

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