floss: fix some rust warnings

* Add serde feature in hcidoc_packets (the generated files has serde
  configuration feature)
* Allow dead code in hcidoc parser (the structs correctly represent
  the actual packet despite the value is never read)
* Don't use DateTime::from_timestamp_micros in hcidoc parser because
  it needs chrono version above what's available on our rust_crates
* Use underscore for default match arm
* Remove unused FromPrimitive trait

Bug: None
Test: m -j
Flag: EXEMPT, floss only changes
Change-Id: Ie81c021b02ca5fa7d142fc64e8cec061c6566f2c