usb: gadget: loopback: don't queue requests to bogus endpoints

A request allocated from e.g. ep1out cannot
be queued to any other endpoint. This bug has
been in f_loopback at least since mid-2011 and
since nobody has really screamed about it, we're
not backporting to any stable kernels.

Debugged with MUSB since that's the only controller
which complains about this case.

Signed-off-by: Felipe Balbi <balbi@ti.com>