usb: dwc3: Don't touch gadget->max_speed while limiting the speed

The gadget->max_speed should not be changed dynamically as it can
mess up the descriptor preparation. Let it be the max supported
by the hardware and changed the dwc3->maximum_speed to force
the controller to enumerate at lower speed when needed. This makes
sure that the descriptors for all speeds are cached during bind
of the function drivers.

Change-Id: Id7fd8f23bb84f7396a6e3b62c5e61c4536522d82
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>