V4L/DVB: media/az6027: handle -EIO failure

If the az6027_usb_in_op() returns a negative errno ret is -EIO and in
that case the value of b[0] may be undefined.  The original code
assigned 0 to ret, but since it's already 0 now we can skip that.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>