media: radio: constify pnp_device_id

pnp_device_id are not supposed to change at runtime. All functions
working with pnp_device_id provided by <linux/pnp.h> work with
const pnp_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>