V4L/DVB (5511): Fix 2/3 for bug 7819: demux and dvr

fixing hotplug issue for demux[n] and dvr[n]

Signed-off-by: Michal CIJOML Semler <cijoml@volny.cz>
Signed-off-by: Markus Rechberger <markus.rechberger@amd.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>