usb: dwc3: gadget: put link to U0 before Start Transfer

Synopsys Databook says we should move link to U0
before issuing a Start Transfer command. We could
require the gadget driver to call
usb_gadget_wakeup() however I feel that changing all
gadget drivers to keep track of Link State and
conditionally call usb_gadget_wakeup() would be far
too much work. For now we will handle this at the
UDC level, but at some point composite.c should be
one handling this.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>