drivers: udc_stm32: handle ZLP flag for control transfers as well

I mistakenly assumed in the commit 6aaad0a5
("drivers: udc_stm32: handle ZLP flag") that the HAL driver would handle
ZLP flag in control transfers itself, but that does not seem to be the
case.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>