media: ti-vpe: cal: Store struct device in cal_dev

The cal_dev structure stores the platform_device pointer, but most
accesses to that field need the device pointer. Store the struct device
pointer directly to simplify the code, and use to_platform_device() in
the two locations that need the platform device.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>