Unify assign_ep_address and usb_gadget_ep_match_desc

[xairy]

Unify checks in assign_ep_address in examples with the ones in usb_gadget_ep_match_desc.

[xairy]

Once this is done, we also need to teach USB Proxy about dynamic endpoint assignment.

[xairy]

Right now the missing checks are:

1. The ones that depend on gadget->max_speed. To implement these, we need to resolve #41 first.

2. Checks for isochronous endpoints. For these, we need to implement handling of isochronous endpoints in Raw Gadget first.