usb: gadget: audio: remove c->highpseed = true from f_midi and uac1

Whether highspeed configuration is valid or not is something that
composite decides and not the gadget. That gadget can only provide the
required descriptors for it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>