V4L/DVB (4392): Fix dst_ca attach

Move the call to dst_attach into the dst_attach function to eliminate
problems caused with dvb_attach.

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