V4L/DVB (4382): Dvb_attach modifications to dvb frontend structures

Add write() op
Add release_sec() op
Add change misc_priv->sec_priv data field
Acked-by: Michael Krufky <mkrufky@linuxtv.org>
Acked-by: Trent Piepho <xyzzy@speakeasy.org>

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