V4L/DVB (4391): Refactor dvb_detach calls into a single dvb_frontend_detach function.

Remove buggy dvb_detach() macro and replace with unified
dvb_frontend_detach() call.

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