usb_dc_stm32: fix init on soft reset

Soft resetting a STM32 device currently does not reset the USB
connection, which causes a few problems: The endpoints do not respond
anymore, and within zephyr any kind of status information like
CDC_SET_CONTROL_LINE_STATE is also missing as they are not
re-negotiated.

This is fixed by stopping the USB device from zephyr side, right after
initialising the USB device.

Signed-off-by: David Jablonski <dayjaby@gmail.com>