V4L/DVB (4854): Handle errors from input_register_device()

Also sprinkled some input_sync() throughout the code.
Acked-by: Ricardo Cerqueira <v4l@cerqueira.org>
Acked-by: Oliver Endriss <o.endriss@gmx.de>
Acked-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>