[media] dvb-usb: silence an uninitialized variable warning

My static checker complains that if adap->props.num_frontends is 0 then
"ret" is uninitialized.  I don't think that can happen.  But "ret" is
always zero here so we can just remove the condition.

This extra check was added in commit 0d3ab841 ('[media] dvb core:
must check dvb_create_media_graph()').

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>