Fully resetting the controller is a too big hammer, and the musb_core will
then afterwards fail to communicate with any endpoints other then 0 as
too much state was cleared.

Instead report vbus low for 200ms which will effectively end the current
session without needing to do a full reset.

This fixes usb mass-storage devices no longer working after a "usb reset"

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>