V4L/DVB (11334): pvrusb2: Fix uninitialized tuner_setup field(s)

Any time a struct (especially one not defined by this driver) is
allocated, we MUST zero its underlying storage.  This makes our usage
of the struct predictable and robust against future changes where
fields might be added that we don't know about.  Failing to do this
with tuner_setup left the config field uninitialized which then caused
trouble with the tuner type used for HVR-1950 devices.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>