V4L/DVB (3741): Budget-core doesn't check if register adapter fails

If you have more than DVB_MAX_ADAPTERS in your machine, this causes an oops.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>