media: v4l: use WARN_ON(1) instead of __WARN()

__WARN() cannot be used in portable code, since it is only
available on some architectures and configurations:

drivers/media/platform/pxa_camera.c: In function 'pxa_mbus_config_compatible':
drivers/media/platform/pxa_camera.c:642:3: error: implicit declaration of function '__WARN'; did you mean '__WALL'? [-Werror=implicit-function-declaration]

The common way to express an unconditional warning is WARN_ON(1),
so let's use that here.

Fixes: 97bbdf02 ("media: v4l: Add support for CSI-1 and CCP2 busses")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>