usb: gadget: u_uac1: Kill set_fs() usage

With the new API to perform the in-kernel buffer copy, we can get rid
of set_fs() usage in this driver, finally.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>