drivers: udc: stm32: fix DOUT stage handling for `st_stm32_usb`

In the device that has `DT_DRV_COMPAT` equal to `st_stm32_usb`,
its behavior differs from `st_stm32_otghs` and `st_stm32_otgfs`
due to the underlying `HAL_PCD_IRQHandler`.
As a result, calling `usbd_ctrl_feed_dout` for the DOUT stage is not
compatible with the `st_stm32_usb` device.

Instead of calling `usbd_ctrl_feed_dout`, we still require flushing
the TX FIFO to the host.
This is achieved by calling `HAL_PCD_EP_Receive` with `len` = `0`.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>