media: dvb-bt8xx: Make variable dst_config constant

Static structure dst_config, of type dst_config, is not used except to
be assigned as the value of field state of a variable having type
dst_state *. In the definition of dst_state, field config is declared as
const. Hence dst_config, when assigned to config, cannot be modified.
Therefore, make dst_config const as well.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>