usb: gadget: core: introduce ->udc_set_speed() method

Sometimes, the gadget driver we want to run has max_speed lower than
what the UDC supports. In such situations, UDC might want to make sure
we don't try to connect on speeds not supported by the gadget
driver (e.g. super-speed capable dwc3 with high-speed capable g_midi)
because that will just fail.

In order to make sure this situation never happens, we introduce a new
optional ->udc_set_speed() method which can be implemented by
interested UDC drivers.

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