`_build_register` should be updated to work with registers with length > 1

[jsiegle]

_Emitter._build_register doesn't currently propagate the register's length onto the class it builds:

python/src/packages/harp-device/src/harp/device/schema/_emit.py

Lines 482 to 490 in 5991c3f

	return _new_class(
	class_name,
	(RegisterBase,),
	{
	"address": reg.address,
	"payload_type": ProtoPayloadType[reg.type.name],
	"payload_class": payload_cls,
	},
	)

This breaks Device.read() and Device.write() for registers with length > 1, such as the struct-typed settings registers used on the Quac.

Changing the return value to this seems to fix things:

  return _new_class(
      class_name,
      (RegisterBase,),
      {
          "address": reg.address,
          "payload_type": ProtoPayloadType[reg.type.name],
          "payload_class": payload_cls,
          "length": length if length > 1 else None,
      },
  )