usb: gadget: u_audio: remove caching of stream buffer parameters

There is no necessity to copy PCM stream ring buffer area and size
properties to UAC private data structure, these values can be got
from substream itself.

The change gives more control on substream and avoid stale caching.

Fixes: 132fcb46 ("usb: gadget: Add Audio Class 2.0 Driver")
Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>