[media] si4713: cleanup platform data

Remove unreferenced members from the platform
data's structure.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>