F4 usb reconnect fix

This enables VBUS_SENSE in the usb driver so we get proper disconnect callbacks.
That probably means usb vsense has to be connected to special function pin.
As far as i know this should be the case on all f4 hardware. (haven't looked up though)

Also removes DCD_EP_Flush on endpoint TX which seems to have been the root cause.
For me it fixes the problem that usb was no longer usable after a reconnect as well as
a periodic short lag in usb data transfer.

Please test on as much hardware as possible and on all platforms.
I had success with DiscoveryF4 and quanton on linux so far.

Supposed to fix #257 and be a part of #146