[media] dvb_frontend: Fix a regression when switching back to DVB-S

There are some softwares (Kaffeine and likely xine) that uses a
DVBv5 call to switch to DVB-S2, but expects that a DVBv3 call to
switch back to DVB-S. Well, this is not right, as a DVBv3 call
doesn't know anything about delivery systems.

However, as, by accident, this used to work, we need to restore its
behavior, in order to avoid regressions with those softwares.

Reported on this Fedora 16 bugzilla:
	https://bugzilla.redhat.com/show_bug.cgi?id=812895



Reported-by: Dieter Roever <Dieter.Roever@gmx.de>
Cc: stable@kernel.org # for version 3.3
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>