usb: gadget: uvc: fix interval_duration calculation

According to USB specification:

  For full-/high-speed isochronous endpoints, the bInterval value is
  used as the exponent for a 2^(bInterval-1) value.

To correctly convert bInterval as interval_duration:
  interval_duration = 2^(bInterval-1) * frame_interval

Because the unit of video->interval is 100ns, add a comment info to
make it clear.

Fixes: 48dbe73 ("usb: gadget: uvc: set req_size and n_requests based on the frame interval")
Cc: stable@vger.kernel.org
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Link: https://patch.msgid.link/20260113-uvc-gadget-fix-patch-v2-2-62950ef5bcb5@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Xu Yang

drivers/usb/gadget/function/uvc.h

@@ -107,7 +107,7 @@ struct uvc_video {
 	unsigned int width;
 	unsigned int height;
 	unsigned int imagesize;
-	unsigned int interval;
+	unsigned int interval;	/* in 100ns units */
 	struct mutex mutex;	/* protects frame parameters */
 
 	unsigned int uvc_num_requests;

drivers/usb/gadget/function/uvc_video.c

@@ -499,7 +499,7 @@ uvc_video_prep_requests(struct uvc_video *video)
 {
 	struct uvc_device *uvc = container_of(video, struct uvc_device, video);
 	struct usb_composite_dev *cdev = uvc->func.config->cdev;
-	unsigned int interval_duration = video->ep->desc->bInterval * 1250;
+	unsigned int interval_duration;
 	unsigned int max_req_size, req_size, header_size;
 	unsigned int nreq;
 
@@ -513,8 +513,11 @@ uvc_video_prep_requests(struct uvc_video *video)
 		return;
 	}
 
+	interval_duration = 2 << (video->ep->desc->bInterval - 1);
 	if (cdev->gadget->speed < USB_SPEED_HIGH)
-		interval_duration = video->ep->desc->bInterval * 10000;
+		interval_duration *= 10000;
+	else
+		interval_duration *= 1250;
 
 	nreq = DIV_ROUND_UP(video->interval, interval_duration);