[media] si4713: add the missing RDS functionality

Not all the RDS features of the si4713 were supported. Add
the missing bits to fully support the hardware capabilities.

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