usb: gadget: f_uac1: silence an info leak warning

Smatch complains that "len" could be larger than the sizeof(value)
so we could be copying garbage here.  I have changed this to match
how things are done in composite_setup().

The call tree looks like:
  composite_setup()
  --> f_audio_setup()
      --> audio_get_intf_req()

composite_setup() expects the return value to be set to
sizeof(value).

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>