[media] si4713: use managed memory allocation

Introduce the usage of managed memory allocation to
simplify the code slightly.

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>