usb: dwc3: workaround: bogus hibernation events

Revision 2.20a of the core has a known issue
which would generate bogus hibernation events
_and_ random failures on USB CV TD.9.23 test
case.

The suggested workaround is to ignore hibernation
events which don't match currently connected
speed.

Signed-off-by: Felipe Balbi <balbi@ti.com>