media: i2c: adv748x: Use devm to allocate the device struct

Switch to devm_kzalloc() when allocating the adv748x device struct.

The sizeof() is updated to determine the correct allocation size from
the dereferenced pointer type rather than hardcoding the struct type.

[Kieran: Change sizeof() to dereference the pointer type]

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>