usb: gadget: function: acm: make f_acm pass USB20CV Chapter9

During Halt Endpoint Test, our interrupt endpoint
will be disabled, which will clear out ep->desc
to NULL. Unless we call config_ep_by_speed() again,
we will not be able to enable this endpoint which
will make us fail that test.

Fixes: f9c56cdd

 (usb: gadget: Clear usb_endpoint_descriptor
	inside the struct usb_ep on disable)
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Felipe Balbi <balbi@ti.com>