[media] Mantis: use dvb_attach to avoid double dereferencing on module removal

Convert the driver to use the dvb_attach macro to avoid the hard dependency
on the frontend drivers.  The hard dependecy will result in loading a
number of unused frontends, and unwanted automatic dereferencing.

This fixes a bug where unloading the mantis driver will derefence any
attached frontend twice, which will cause an oops if the same frontend is
used by another driver.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>