usb: gadget: audio: queue wLength-sized requests

On Audio class, the wLength field of the Setup
packet, contains the data payload size of the
following Data phase. Instead of harcoding values,
use wLength.

This also fixes a bug where Gadget driver had to
receive 3 bytes, but it was queueing a ZLP.

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