usb: dwc3: make sure UX_EXIT_PX is cleared

This bit is only supposed to be used with known
buggy PHYs, however some platforms might erroneously
set it. In order to avoid it, let's make sure this
bit is always cleared. If some PHY needs this, we
will need to add a quirk flag.

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