usb: gadget: remove DMA_ADDR_INVALID from f_uac2 and gadgetfs

DMA_ADDR_INVALID is used by the UDC driver and the gadgets should
provide only a buffer address. Everything else should be taken core of
by the UDC and udc-core.

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