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 <[e-mail]>
Signed-off-by: Jingoo Han <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/u_uac1.c

···
 	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);