usb: gadget: u_uac1: add __user annotation

Added __user annotation to fix the following sparse warning.

drivers/usb/gadget/u_uac1.c:194:52: warning: incorrect type in argument 2 (different address spaces)
drivers/usb/gadget/u_uac1.c:194:52:    expected void const [noderef] <asn:1>*buf
drivers/usb/gadget/u_uac1.c:194:52:    got void *buf
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/u_uac1.c

@@ -191,7 +191,7 @@ static size_t u_audio_playback(struct gaudio *card, void *buf, size_t count)
 	frames = bytes_to_frames(runtime, count);
 	old_fs = get_fs();
 	set_fs(KERNEL_DS);
-	result = snd_pcm_lib_write(snd->substream, buf, frames);
+	result = snd_pcm_lib_write(snd->substream, (void __user *)buf, frames);
 	if (result != frames) {
 		ERROR(card, "Playback error: %d\n", (int)result);
 		set_fs(old_fs);