stm32/usbdev: Fix compile error if MICROPY_HW_USB_CDC_NUM is set to 2.
stm32/usbdev: Fix compile error if MICROPY_HW_USB_CDC_NUM is set to 2.

Fixes regression introduced by 6705767da1f7dba7a04e1d16c380a650f1f1074f

trezor/micropython,selste/micropython,henriknelson/micropython,bvernoux/micropython,pramasoul/micropython,pramasoul/micropython,adafruit/circuitpython,trezor/micropython,pramasoul/micropython,MrSurly/micropython,trezor/micropython,tobbad/micropython,pozetroninc/micropython,pozetroninc/micropython,pramasoul/micropython,trezor/micropython,MrSurly/micropython,bvernoux/micropython,henriknelson/micropython,MrSurly/micropython,henriknelson/micropython,selste/micropython,pozetroninc/micropython,adafruit/circuitpython,kerneltask/micropython,pozetroninc/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,selste/micropython,pozetroninc/micropython,trezor/micropython,henriknelson/micropython,MrSurly/micropython,kerneltask/micropython,tobbad/micropython,selste/micropython,henriknelson/micropython,MrSurly/micropython,selste/micropython,kerneltask/micropython,adafruit/circuitpython,adafruit/circuitpython,kerneltask/micropython,pramasoul/micropython,bvernoux/micropython,kerneltask/micropython,tobbad/micropython,bvernoux/micropython,tobbad/micropython,tobbad/micropython