[media] si4713: si4713_set_rds_radio_text overwrites terminating \0

si4713_set_rds_radio_text will overwrite the terminating zero at the
end of the rds radio text string in order to send out a carriage return
as per the RDS spec.
Use a separate char buffer for the CR instead of corrupting the control
string.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Eduardo Valentin <edubezval@gmail.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>