usb: gadget: tcm_usb_gadget: stop format strings

commit aba37fd975f0dd58e025c99c2a79b61b20190831 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>