serial: Refactor slightly to improve throughput
serial: Refactor slightly to improve throughput

It's unclear whether this refactor improves things at all, but it's
still limited to 32-byte buffers.  Still, it results in less code,
which seems like a good thing.

Signed-off-by: Sean Cross <dd28e116ed3e781f3ae781acae9fb18383f35c2f@kosagi.com>

pfalcon/fernly,mandl/fernly,urjaman/fernly,mandl/fernly,urjaman/fernly,xobs/fernly,xobs/fernly,urjaman/fernly,pfalcon/fernly,mandl/fernly