[media] dvb_frontend: fix a regression with DVB-S zig-zag

[media] dvb_frontend: fix a regression with DVB-S zig-zag

Changeset 5bfaadde broke zig-zag for DVB-S drivers that don't
implement get_tune_settings() callback.

Fix the code, in order to allow it to work as before, otherwise
some channels may not be tuned anymore.

Fix Fedora Bugzilla:
	https://bugzilla.redhat.com/show_bug.cgi?id=814404

Cc: stable@kernel.org # for Kernel v3.3
Reported-by: Michael Heijenga <database.worker@googlemail.com>
Tested-by: Michael Heijenga <database.worker@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>