usb: gadget: tcm_usb_gadget: stop format strings

This makes sure that the name coming out of configfs cannot be used
accidentally as a format string.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>