usb: dwc3: take lock while modifying flags

dwc3_gadget_ep_set_wedge() and dwc3_gadget_set_selfpowered() were
modifying dwc->flags/dwc->is_selfpowered without taking the lock.
Since those modifications are non-atomic, that could cause other
flags to be corrupted. Fix them both to take the lock.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>