[media] Fix regression in some dib0700 based devices

Fix regression in some dib0700 based devices.
Set size_of_priv, and don't call dvb_detach unnecessarily.
This resolves the oops(s) for my "Leadtek Winfast DTV Dongle (STK7700P based)"

Signed-off-by: James Harper <james.harper@ejbdigital.com.au>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>