usb: gadget: f_uac2: fix compile warning

this warning was introduced by previous patches,
fix it.
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_uac2.c

@@ -260,7 +260,6 @@ static int
 uac2_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
 {
 	struct snd_uac2_chip *uac2 = snd_pcm_substream_chip(substream);
-	struct audio_dev *agdev = uac2_to_agdev(uac2);
 	struct uac2_rtd_params *prm;
 	unsigned long flags;
 	int err = 0;