[media] media: adv7604: automatic "default-input" selection

Documentation states that the "default-input" property should reside
directly in the node of the device.  This adjusts the parsing to make the
implementation consistent with the documentation.

Based on patch by William Towle <william.towle@codethink.co.uk>.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>