Fix issue where the data register may be overwritten by next character
reception before being read and returned.

Signed-off-by: Olivier Hériveaux <olivier.heriveaux@ledger.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211128120723.4053-1-olivier.heriveaux@ledger.fr
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>