V4L/DVB (8040): soc-camera: remove soc_camera_host_class class

Devices can either be class devices or bus devices, not both at the
same time. Soc-camera host devices usually have a platform device as
their parent. Trying to also register them with a class crashes the
kernel, when linked statically. Interestingly, it works when built
as a module. Thanks to Paulius Zaleckas for reporting.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>