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

[ Upstream commit 52ec49a5 ]

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)
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>