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>