V4L/DVB (5630): Dvb-core: Handle failures to create devices

dvb-core is not started early enough when device drivers that use dvb are
compiled in so dvb_register_device fails (silently) since dvb_class is
NULL, this runs dvb_init using subsys_initcall instead of module_init.

dvb_register_device will now check the return value of class_device_create.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>