usb: usb_dc_stm32: Add placeholders for missing API

Fix build error when executing sanitycheck on stm32 devices.

Command to execute tests:
sanitycheck --device-testing --device-serial /dev/ttyACM0 -p <board>
    -t usb

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>