USB: Replace own str_plural with common one

Use existing str_plural() helper from string_choices.h to reduce amount
of duplicated code.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250114-str-enable-disable-usb-v1-1-c8405df47c19@linaro.org


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