When used as EXTI inputs, GPIO pins can only generate edge triggered interrupts. You are going to have to either add your own software debounce or change your hardware. You could detect an edge, then disable the EXTI interrupt for some debounce interval, sample the input to see what state it is in, then re-enable the EXTI. Another alternative may be to simply poll the pin in your SYSTick interrupt and do software debounce. Or use another timer if you need faster sampling (you shouldn't, not for USB connect/disconnect events). Or, maybe, depending which GPIO pin you are using, there may be an alternate function as an input to a peripheral that you may be able to use to simulate a level sensitive interrupt. Off the top of my head the only one I can think of would be to use the input to gate a timer, and if the timer is enabled to count some number of counts declare the signal "asserted" and then change the input polarity to detect the other transition. Look for "slave mode gated mode" in the timer section of the reference manual.
