usb: gadget: u_audio: remove cached period bytes value

Substream period size potentially can be changed in runtime, however
this is not accounted in the data copying routine, the change replaces
the cached value with an actual value from substream runtime.

As a side effect the change also removes a potential division by zero
in u_audio_iso_complete() function, if there is a race with
uac_pcm_hw_free(), which sets prm->period_size to 0.

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>