USB: core: export usb_cache_string()

usb_cache_string() can also be useful for the drivers so export it.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/all/20221130174658.29282-4-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>