CAN: Fix canbusutil receive flag display

Use a mutable QString for the BRS, ESI, and local echo flags.
QLatin1String returns characters by value, so assigning through its
subscript operator only changes discarded temporaries and leaves the
display unchanged.

Broken by: 38def2c0817291c09c440c074f011c4f684eaccf

[ChangeLog][CAN][canbusutil] The --info option now displays the
bitrate switch, error state indicator, and local echo flags correctly.

Pick-to: 6.12 6.8
Assisted-by: OpenAI codex (gpt-6.1-sol)
Change-Id: I38f1c18b5ec59b4476d060fd66869f9e7afc7f8a
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>