usb: informative message if no controller

The message 'No working controllers found' provides no clue that this
refers to USB controllers.

Provide a message that refers to USB.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>