[media] si4713: switch to devm regulator API

This switches back to the normal regulator API (but use
managed variant) in preparation for device tree support.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: fixed two trival compiler warnings]

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>