scripts: west: runners: stm32cubeprogrammer: Fix stm32n6 dfu flashing

Rework of the flashing algorithm broke the STM32N6 dfu flashing.
Add the case of using usb port and download-modifier (new n6 specific
option) as a new case of using the bin file for flashing.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>