usb: builtin handling of interface alternate settings

The stack reads its own descriptors to figure out which endpoints
are used in which alt settings, and enables/disables them as needed.

The ControlHandler has a callback so it can get notified of alternate
setting changes, which is purely informative (it doesn't have to do anything).

embassy-usb/src/descriptor.rs

@@ -1,5 +1,5 @@
 use super::builder::Config;
-use super::{types::*, CONFIGURATION_VALUE, DEFAULT_ALTERNATE_SETTING};
+use super::{types::*, CONFIGURATION_VALUE};
 
 /// Standard descriptor types
 #[allow(missing_docs)]
@@ -192,7 +192,7 @@ impl<'a> DescriptorWriter<'a> {
         interface_protocol: u8,
         interface_string: Option<StringIndex>,
     ) {
-        if alternate_setting == DEFAULT_ALTERNATE_SETTING {
+        if alternate_setting == 0 {
             match self.num_interfaces_mark {
                 Some(mark) => self.buf[mark] += 1,
                 None => {